Let me say up front that I apologize if some readers find the options below unpalatable. They clearly aren't for every table. The question itself already exhausts the usual ways of making additional ranged attacks, and the asker's group's and personal binds further restrict the available options. Also, while the asker's comments indicated that material is acceptable if "it isn't game-breaking or silly," both are subjective: One group's game-breaking or silly can be another's normal.
That said, this answer first relies on a weapon that some groups may think is silly. Also, in a typical campaign with less restrictive binds, the weapon is possibly unbalanced. However, in a campaign that appears to favor casters to the degree that the question's campaigns do, I suspect that the weapon may be met with less incredulity. Still, the weapon mandates a conversation with the group. Then this answer suggests gaining more limbs to wield more ranged weapons. Again, while some groups would be okay with this, the vagueness of the rules, the fighter's appearance, or both may be too much for the group.
Main-"hand" mouth darts instead of a hand crossbow
The exotic weapon fukimi-bari (Arms and Equipment Guide 7) (1 gp; 0.1 lb.) was never subject to the 3.5 revision therefore making it game-legal but subject to minor adjustments by the DM. Assuming those minor adjustments don't radically alter the weapon, when a creature makes an attack, the creature can spit three fukimi-bari. As fukimi-bari are darts that a creature holds in its mouth and spits at its foes, practically, it's unlikely that a DM will allow a creature to two-weapon fight exclusively with fukimi-bari unless the creature's a chimera (Monster Manual 34), ettin (106–7), or another multimouthed creature (but see below).
Thus a typical human can two-weapon fight by spitting fukimi-bari as her main-"hand" weapon and, for example, firing a light crossbow with her off-hands. Fukimi-bari damage is low, so buying a batch of +1 flaming frost shock fukimi-bari for boss battles will probably be a thing. (A Medium creature's normal fukimi-bari deal but 1 point of damage and have a threat range of 20. Also, precision damage only applies to the first 'bari in a 3-round burst. Finally, while they're technically thrown weapons the wielder's Strength modifier doesn't apply to damage.) While damage drops substantially—the feat Crossbow Sniper (Player's Handbook II 77–8) doesn't apply, for instance—the sheer volume of attacks probably compensates, as fukimi-bari triple the number of main-hand ranged attacks a typical human can make.
Buy another mouth to two-weapon fight with just mouth darts
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 themselves say one way or the other whether they replace or supplement existing body parts, no matter how unusual or disturbing the outcome of the latter may be.
And when I say unusual or disturbing, I mean it because, with the above in mind, it's possible that the graft fiendish jaw (210) (2,000 gp; graft) adds to a creature at least a jaw if not an entire mouth! This makes it possible for a fiendish jaw graft to allow an otherwise normal human to two-weapon fight with mouth darts… if the DM agrees, of course.
How 'bout instead just a bunch of swole arms?
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, there's still one that's interesting: the illithid graft rending claw (Fiend Folio 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, the rending claw graft 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, but each additional limb can wield an additional hand crossbow and, with it, make off-hand attacks.
Be warned: These—or any other of a number of different kinds of additional limbs (see here)—causes a typical humanlike creature to enter into the weird rules territory of multiweapon fighting (see, for example, here). Your group may pelt you with dice.
…Or pay to have the fighter transformed into an actual monster
Assuming the fighter can find a friendly wizard to cast upon her the 8th-level Sor/Wiz spell polymorph any object [trans] (Player's Handbook 263) (see here), the fighter can become permanently any creature that possesses no more than 15 Hit Dice that's of the same kingdom and the same or lower Intelligence score and that's either the same class or the same size. For example, an Int 12 fighter may be able to be permanently transformed into a four-armed xill (MM 259–60) or an eight-tentacled tako (Oriental Adventures 193). (Dumpster-diving and the table's definitions of class and kingdom may yield better options.) Note that a duration of permanent is different from a duration of instantaneous, making a permanent-duration polymorph any object spell still susceptible to dispel magic effects. Caution is advised.
Note: I know that I just shouldn't contribute answers with which I'm uncomfortable, and these unconventional suggestions do make me uncomfortable. However, the asker's present binds significantly curtail available options, and, having dispensed with conventional options, only unconventional ones remain. I honestly look forward to answers that present better options than this one does.
