Are there any feats that let you treat tentacles as arms that can be used to wield weapons, a la Prehensile Tail for tails? Alternatively, are there any arm grafts that can add to my total number of arms, either by adding an arm without replacing anything or by replacing a tentacle?
There are no feats that I'm aware of in official or officially licensed products that specifically allow one or more tentacles to be treated like hands for the purposes of wielding weapons. In fact, only a (ahem) handful of feats broach the topic of tentacles—the most interesting likely being the feat Extended Reach (Savage Species 34). In short, feats are not really your go-to here.
To compensate, I can point you to the gloves of man (Savage Species 57) (42,000 gp; 0 lbs.), the description of which, in part, says, "These magic gloves provide humanoid fingers and thumbs for the wearer. Any creature may wear them, provided the creature has tentacles or paws over which to slip them. While wearing the gloves, the creature may manipulate items as if it had fully functional hands." Expensive? Certainly. Nonetheless, despite not being a feat, the gloves do exactly what the question desires.
Wielding weapons with tentacles
Savage Species on Creating an Anthropomorphic Animals says, "An anthropomorphic animal has the natural attacks of the base creature, but it can also use weapons if it did not have hands already," then it continues, saying, "If the base creature has more than two limbs, the anthropomorphic animal has the same number, but two of them become feet. Thus, an octopus-man, for example, would have only six natural attacks rather than eight" (215). I'm not entirely sure how much to read into this description: Savage Species may assume that an anthropomorphic octopus can use its tentacles to wield weapons, that an anthropomorphic octopus has two humanlike legs and six humanlike arms yet retains somehow the tentacle natural attacks tied to those now humanlike arms, or something else entirely. What does seem clear is that there's at least an argument to be made that the game thinks tentacles are already weapon-wielding limbs and that there's no need to take a feat to make them so.
There's also the example of the tako (Oriental Adventures 193), an aberration that has 7 arms according to its stat block but has "eight supple tentacles" in its description. Each of a tako's arms or tentacles or whatever is capable of wielding a weapon.
(Further, the Advanced Dungeons & Dragons, 2nd Edition illustration of the tako shows the creature clearly armed with a variety of weapons rather than just brandishing one.) So talk to the DM and confirm first that there's even a need for a tentacle to be changed by a game element so that it's capable of wielding a weapon as there's a decent argument in favor of there not being such a need.
(By the way, the goal may be to meet the Multiweapon Fighting feat's prerequisite that includes "three or more hands" (Monster Manual 304). If that's the case, urge the DM take into consideration both the Improved Multiweapon Fighting and Greater Multiweapon Fighting feats' prerequisites that include—instead of the earlier feat's hands—"three or more arms" (Savage Species 36 and 35, respectively, and emphasis mine). I know, right?)
Acquiring additional manipulative and semimanipulative limbs
This list is alphabetical. If in a hurry, as they have the most potential for PC use, skip to the sections on Fiendish Grafts and Undead Grafts, paying particular attention to the latter.
Aboleth Grafts: These are almost always incredibly impractical and unsuited for PCs. Aboleth grafts first appeared in the Fiend Folio (208) and were updated to the 3.5 revision by Lord of Madness (215). The Fiend Folio holds the only one of interest here, the aboleth tentacle (FF 208) (50,000 gp; graft) that "typically replaces an arm or forelimb on the grafted creature, though sometimes it is attached just above a forelimb or below an arm." (As the aboleth graft skum tail (ibid.) (6,000 gp; graft) doesn't grant a tail attack, a creature must already have a tail attack to turn the skum tail graft into a manipulator with the feat Prehensile Tail (Serpent Kingdoms 147); getting a tail attack is beyond this question's already prodigious scope.)
However, an aboleth graft that's applied to a creature not created by an aboleth (e.g. a skum (Monster Manual 228)) "dries up and withers over the course of 1d4+1 days, falling off at the end of this period" (LoM 215). The 7th-level Clr spell regenerate [conj] (Player's Handbook 270) can restore the body part the aboleth graft replaced (if any), but even that spell can't restore the aboleth graft itself. (A high-level druid that possesses the feat Aberration Wild Shape (LoM 178) or another creature capable of spending its time constantly in the shape of a skum possibly could—with the DM's permission—probably indefinitely delay its aboleth grafts drying up, withering, and falling off, but that's an enormous, complicated, and precarious investment of resources.)
Fiendish Grafts: A nonevil-aligned creature that's considering a fiendish graft should be aware that for it a fiendish graft's drawbacks are significant. Anyway, the Fiend Folio on Acquiring a Fiendish Graft, in part, says, "Certain magical devices have been discovered that enfold a creature, remove one of its limbs,… and replace the limb with a fiendish graft…" (209), but whether this horrific event also occurs when a fiendish graft is attached conventionally—like, for example, by an O-so-benevolent terrestrial creator—is unstated. None of the fiendish grafts say one way or the other whether they replace or supplement existing body parts, no matter how unusual or disturbing the outcome the latter may be. (Yes, I'm looking at you, fiendish ear (210) (2,500 gp; 0 lbs.), and especially at you, fiendish jaw (210) (2,000 gp; graft), you weirdo.) Ask the DM.
Illithid Grafts: Lord of Madness and Underdark present additional illithid grafts (216 and 77, respectively) that supplement those found in the Fiend Folio (212–13). Possessing even one illithid graft causes the creature to suffer a −4 penalty on Will saving throws against mind-affecting effects and psionic powers. That said, the only one of interest is the illithid graft rending claw (FF 212–13) (5,000 gp; graft) that is specifically a manipulator (albeit a poor one for fine work) and that does not say specifically that it replaces a limb. However, it is "actually an entire arm—overlong for the grafted creature’s height (typically reaching all the way to the floor) and powerfully muscled," so it might take a few beers to convince the DM that a PC can have, like, nine of them or whatever. (The two tentacle illithid grafts from Underdark are each specifically "grafted to a… recipient in place of its own arm or forelimb" (77). Are one or more of a specific creature's tentacles also its forelimbs? Ask the DM.)
Undead Grafts: Libris Mortis updates for the 3.5 revision the undead grafts that appeared originally in the Fiend Folio (214). The Fiend Folio on Undead Grafts says that "undead grafts are formed of nonliving flesh attached to a still-living body" (ibid.) but, like Libris Mortis, mentions no general rule that says an undead graft replaces an appropriate part rather than the creature simply getting more parts. In fact, the implication is the opposite: a creature gets more parts unless the specific undead graft says it replaces a part. Compare: The undead graft mummified eye (LM 80) (50,000 gp; graft), in part, says, "This hard, round orb fits into a humanoid creature’s empty eye socket," and the undead graft vampire fangs (ibid.) (25,000 gp; graft), in part, says, "This set of sharp teeth replaces the creature’s existing teeth," while the undead graft ghostly arm (ibid.) (6,000 gp; 0 lbs.) says
This gray, incorporeal arm can’t be used to manipulate solid objects. However, the grafted creature can use the ghostly arm to deliver an incorporeal touch attack that deals 1d6 points of damage, the equivalent of a ghost’s corrupting touch attack. Treat the arm as a secondary weapon, but because it is incorporeal the arm has no Strength score and therefore gains no bonus or penalty on damage rolls from the user’s Strength.
Thus, apparently, for instance, a creature can get any number of these gray, incorporeal arms and gain that many incorporeal touch attacks as secondary natural weapons. (For a creature that possesses the extraordinary ability sneak attack like a rogue, this in particular is a pretty sweet deal!) (Other limbs, of course, have different specific rules on how they're used.)
Although one might think there'd be some issues with having a chunk of an undead creature cleaved to one's living form, undead grafts don't generally cause the creature problems. Narratively, there might be some issues (e.g. Whose arm was this?, How many innocent folks has this arm slain?, Is this arm evil, and, if it is, am I also evil?… that sort of thing), but, mechanically, having a mummy arm or whatever? No big deal.
Yuan-ti Grafts: Those that originally in the Fiend Folio (214–15) were updated to the 3.5 revision by Serpent Kingdoms (154), and includes this brief description: "Yuan-ti grafts are serpentine body parts that can be attached to the body of another corporeal creature," with other details explaining who attaches them (i.e. usually—surprise!—yuan-ti) and who gets them (i.e. usually—surprise!—yuan-ti allies) but not if such grafts generally replace or supplement existing parts. A creature suffers no particular drawbacks upon acquiring one or more yuan-ti grafts, but a creature that possesses at least one yuan-ti graft counts as a serpentfolk (5), a nonsubtype designation that nonetheless carries some weight (q.v. feats like Puff Torso (147), answers to this question).
Of interest is the serpent arm (154) (6,000 gp; graft) that's a "long, flexible arm [that] ends in a snake head." Because the arm has a mouth, a DM may allow it to wield a magic weapon that possesses the magic weapon special ability mouthpick (LoM 46) (+1 bonus; 0 lbs.), although a stricter DM may forbid it due to that magic weapon special ability's technical specifications. (The yuan-ti graft added tail (ibid.) (6,000 gp; graft), despite granting the creature the special attack constrict, still doesn't grant a proper tail attack (q.v. Aboleth Grafts, above).) (The serpent arm graft also might be susceptible as per this question.)
There are no new manipulative or semimanipulative limbs available as beholder grafts (FF 208–9), inevitable grafts (from the Dragon #315 article "Planescape: The Exiled Factions" 46 but fleshwarpers shouldn't get their hopes up as there are only two and they're both pointless and overpriced), maug grafts (FF 213–14 and acquiring them are narratively complicated anyway), pirate grafts (from the Dragon #318 article "X Marks the Spot: Setting Sail with Saber and Spell" 53–4), or silthilar grafts (LoM 215–16). Magic of Eberron includes deathless grafts (127–9), elemental grafts (129–34), and plant grafts (134–6), but these kinds of grafts—that, by the way, follow a much stricter and unified ruleset—offer neither new nor replacement limbs.
In addition to the aforementioned arms of the naga (SS 55) (56,000 gp; 8 lbs.), consider the spare hand (Magic Item Compendium 137) (12,000 gp; 0 lbs.) and using the Improving Magic Item rules (233) to pay 18,000 gp to add a spare hand to your spare hand and so on. With a successful Use Magic Device skill check to emulate a level 5 artificer's class feature infusions (DC 25), a wearer of a spare hand should be able to convince the hand that its been powered by a 3rd-level infusion à la Lidda expending turn attempts she doesn't have to activate the magic chalice (PH 86). Clear this with the DM, though, and make the DM aware that there could be significant fallout from saying yes (e.g. belt of the wide earth (204) (8,000 gp; 0 lbs.), vestments of ebonsilk (147) (5,000 gp; 1 lb.)).
Also, compare the spare hand with the replacement arm for good-aligned creatures, the arm of Nyr (Defenders of the Faith 26) (12,800 gp; 0 lbs.). This player suspects most DMs would balk at a player trying to add to his PC's arm of Nyr a second, third, or ninety-eighth arm of Nyr, but it may be worth an ask.
Note: The writer extends his heartfelt apologies to the biologists who read this answer and yelled at their screens, "Arms have suckers along their lengths while tentacles have suckers just on their ends!" By necessity, this answer uses game terms not biology terms.
Savage Species (p. 55) introduces the Arms of the Naga:
These beautiful, silver arms hang from a harness made from top-grade leather. The arms are proportioned as if for a Medium-size humanoid. They have two "fingers" and an opposable "thumb" on each hand. The magic that gives life to the arms when worn duplicates the Strength and Dexterity of the character wearing them. In a stressful or demanding situation, such as combat, the wearer must make a Will save (DC 19) or take a –2 penalty on all attack rolls, saves, skill checks, and ability checks until the situation passes. Magic armor shifts to accommodate the new arms.
Arms of the naga do not allow additional attacks or the performance of several simultaneous physical tasks. They do not increase the wearer's reach. However, they do allow the wearer to use items designed for creatures with humanoid hands and arms. If the wearer already has arms, arms of the naga allow the use of a two-handed weapon plus another piece of equipment. For instance, a wearer could wield a greatsword while still using a shield.
Arms of the naga have hardness 10 and 30 hit points each. A character cannot remove arms of the naga from an opponent by succeeding on a disarm attempt. The arms are donned or removed like medium armor and are subject to attacks that target armor, such as a bebelith's armor damage attack, but require no proficiency to use.
Caster level: 7th; Prerequisites: Craft Wondrous Item, polymorph other; Market Price: 56,000 gp. Weight: 8 lb
However, personally I would rule that if a creature with tentacles has proficiency with a weapon, they are capable of holding it. In real life, tentacles are quite prehensile and can hold things, after all.