There are a few options, but none of them are pretty
As the 7th-level Clr and 8th-level Sor/Wiz spell greater plane shift [conj] (Spell Compendium 159) appears in only two sources—originally it's from the Planar Handbook—, it would be highly unusual to find in other sources game elements that generate an effect like it. The 3.5 editorial mandate was, generally, to assume that a consumer possessed only the core rules and the text involved, and there is no other mention in the PlH or the SpC of the spell greater plane shift. For example, no magic item has it as a prerequisite, no cleric domain includes it, no prestige class has it by name on its unique spell list, and no touchstone site grants it specifically as base ability or higher-order ability. As this fine answer mentions, even a deity that uses to traverse the planes its divine characteristic travel (Deities & Demigods 30) ends up embarrassingly off-target! (Don't worry about that god, though—it deity can just teleport to the desired destination next round.)
This means you'll probably be stuck with the two-spell combination. Below I've listed some possibilities for achieving that combination at will.
- A level 9 nomad with a power point recharge trick (e.g. here) can employ a combination of the 5th-level nomad power psionic plane shift [psychoportation] (Expanded Psionics Handbook 124) and the 5th-level psion/wilder psionic teleport [psychoportation] (136) to effectively accomplish what the question asks. From personal experience, such tricks aren't all theoretical: I have been both a DM of a campaign and a player in a different campaign wherein power point recharge tricks were allowed, and I didn't find the campaigns any more broken than the Dungeons & Dragons 3.5 campaigns that forbade them. Still, while a few power point recharge tricks are technically legal, I suspect many DMs ban power point recharge tricks anyway, even though most require a significant investment of character resources.
Like this fine answer mentions, some creatures possess a usable-at-will supernatural ability that's like the spell plane shift (e.g. a bebilith (Monster Manual 42)) or greater teleport (e.g. a creature that possesses the subtype archon (16, 306)). A creature that can change its form into one or more of these creatures can take the feat Assume Supernatural Ability (Savage Species 30–1) and pick such an ability so as to use that ability at will while in the new form.
For example, a level 12 sorcerer (who's type is probably outsider) that can cast the 4th-level Sor/Wiz spell polymorph [tran] (PH 263) could take the feat twice—once for each ability—and satisfy the question's requirements, but that sorcerer likely must find a psion to use on her the 4th-level psion power psychic reformation [telepathy] (XPH 127) if the desire is to do this at this low a level. Alternatively, the sorcerer could just wait until she can cast the 9th-level Sor/Wiz spell shapechange [trans] (PH 277–8) as that spell by default grants its subject an assumed form's supernatural abilities, no feats necessary.
However, neither option in isolation actually accomplishes what the question asks. While the new form allows the creature to employ at will the effects the question wants, the method the creature uses to assume that form likely has use limits of its own (but see also, for example, the phylactery of change (Arms and Equipment Guide 135) (11,200 gp; 0 lbs.) and the skin of proteus (XPH 176) (84,000 gp; 2 lbs.)).
A level 14 PC can spend the majority of her wealth on an amulet of the planes (Dungeon Master's Guide 247) (120,000 gp; 0 lbs.) that, in part, says that it
allows its wearer to utilize plane shift. However, this is a difficult item to master. The user must make a DC 15 Intelligence check in order to get the amulet to take her to the plane (and the specific location on that plane) that she wants. If she fails, the amulet transports her and all those traveling with her to a random location on that plane (01–60 on d%) or to a random plane (61–100).
(Emphasis mine.) Ask the DM if the amulet's description means that a successful Intelligence check transports the wearer with perfect accuracy. That is, the DM may rule that a successful check takes the wearer both to the desired plane and to "the specific location on that plane," the description making an exception to the normal operation of the 7th-level Sor/Wiz spell plane shift [conj] (Player's Handbook 262). Alternatively, the DM may rule that the effect otherwise works identically to the spell plane shift therefore still depositing the wearer a woeful 5d% miles from the desired specific location on the picked plane despite a successful check. (To be clear, it's mainly the amulet's price that supports the case for imprecise arrival.)
(By the way, it could be unintentional, but the amulet's description says that the amulet "allows its wearer to utilize plane shift" yet does not mandate the wearer use the effect on herself! Instead, the wearer can use a plane shift effect except that she must succeed on the Intelligence check to use the amulet to take her to the proper destination. It seems that the wearer can try to send another creature but not itself to other planes normally—that is, by touching that other creature—, and no Intelligence check's needed. Again, though, before investing 120,000 gp in an amulet of the planes for this purpose, run this by the DM.)
As long as the creature can get to the destination a different way once, drop off an object safely, and confirm that the object will remain undisturbed at that destination, the creature can drop off at the destination one moving portal as per the feat Create Moving Portal (Perilous Gateways Web column "Portals of Anauroch: At'ar's Portal Collection"). Then the creature can use a second moving portal that leads to the first to travel at will to that first portal. Each moving portal has a market price of 125,000 gp, so a pair is only available to level 16 or higher PCs.
(A keyed moving portal has the same price and can prevent unauthorized use, and it's likely the user will want the moving portal he's not carrying to be keyed! Also, this method assumes that it's dangerous or impractical to spend 3 months or so creating an actual fixed portal on another plane, but, y'know, if Diirinka is okay with the creator wintering in Hidden Betrayal or whatever, creating a normal nonmoving destination portal shaves 25,000 gp off the market price.)
A level 21 character—usually a warlock—that meets its prerequisites can take the epic feat Dark Transient (Epic Insights Web column "Epic Warlock Feats") to gain the ability to
use plane shift with perfect accuracy (self and personal equipment only). [The warlock] can also transport other creatures, but [the warlock will] then suffer the inaccuracies normally incurred when using this spell [so work around this by having party members travel with the warlock in the warlock's portable hole]. Activating this ability requires a standard action.
With such an ability, it's largely irrelevant for the question's purposes that the Dark Transient feat also grants an at-will effect similar to the greater teleport spell.
As this fine answer mentions, an epic hellbreaker (Fiendish Codex I 86–9) or an epic spellthief (Complete Adventurer 13–20) could "borrow" from a familiar or another ally a spell-like ability that's like the greater teleport or plane shift spell. This will likely be more difficult for a hellbreaker on her own, but a spellthief will probably have an improved familiar that could possess such a spell-like ability, albeit probably not both (see here).
By way of ancillary options, I would be remiss were I not to mention the figurine of wondrous power (obsidian steed) (DMG 257) (28,500 gp; 0 lbs.) that can be activated for 24 hours 1 day per week and that's like a heavy warhorse "with the following additional powers usable once per round at will: overland flight, plane shift, and ethereal jaunt," the latter two also affecting the rider. I don't know in this context what a power is either, but a stable of seven obsidian steeds, while pricey, is certainly stylish, even if the rider must still provide her own teleport effect (or just reach the destination conventionally via flying horse).
Finally, a nightmare (MM 194–5) can at will employ as a spell-like ability an effect like the spell astral projection as a level 20 caster, and that spell voids the need for the combination altogether. A level 5 blackguard (DMG 181–3) (I know, right? And, so far as I'm aware, only a blackguard!) can replace her fiendish servant with a nightmare by taking the feat Nightmare Steed (Polyhedron #159 23).
Note: With Wizards of the Coast's sudden but inevitable removal of its online 3.5 repository, links to this answer's Web articles can't be provided easily. I hope enough information is given here so that an interested reader can hunt down those Web articles on another site.