Without using the Complete Warrior prestige classes drunken master (27) and hulking hurler (40), the former which is designed for improvised weapon using monks and the latter which is designed to throw improvised moons, improvised weapons don't get a lot of love in the hardbacks.
However, Phillips-Watts's article "The Way of the Fist: 3 Prestige Classes" in Dragon #295 introduces the Improvised Weapon Proficiency feat on page 73, which, despite the name, only reduces the improvised weapon nonproficiency penalty from -4 to -2. The feat has no prerequisites. The article includes the brawler prestige class which uses improvised weapons as one of its schticks.
In City of Stormreach on pages 99-102 there's the affiliation Bloodbound of the Red Ring which, after joining (requiring role-playing plus 3 ranks in either the skill Intimidate or the skill Perform) and attaining the awesome title of Slaughter Master at an affiliation score of 11, as one of its benefits reduces the penalties by 2 to use improvised weapons. Sometimes, reading these books, it's like the author had someone in his game with exactly that problem and wrote the text to solve it. One can gain an affiliation score of 11, thus becoming a Bloodbound Slaughter Master of the Red Ring (which is so metal I just got mercury poisoning), pretty easily after level 9 or so when one can gain +1 to one's affiliation score by "slay[ing a] fierce monster in the ring" (100) that's CR 8.
The second-party Races of Ansalon from Margaret Weis Productions includes both the feats Improvise Weapon and Greater Improvise Weapon, with the former's prerequisite 1 rank in the Appraise skill, and the latter's prerequisite the former feat. The first is bad-bad (compared to the feat printed above, anyway), forcing a full-round action to reduce 1 improvised weapon's penalties by 2 (albeit for you forever), but the second feat has no such limitation and just negates the improvised weapon penalty completely, so that's okay.
That's it for fighter options.
But Drain's article "Class Acts--Warrior: Law and Chaos" in Dragon #349 introduces on page 92 the variant class feature city brawler for barbarians that removes martial weapon proficiency, medium armor proficiency, and shield proficiency, but grants the feat Improved Unarmed Strike as a bonus feat, the benefits of the feat Two-weapon Fighting when fighting unarmed, and the benefits of the feats Improved then Greater Two-weapon Fighting at levels 6 and 11, respectively. What's of more interest is the ill-placed final paragraph which reduces improvised weapon penalties from -4 to -2; I say ill-placed because a strange and silly argument could ensue that it's unclear the city brawler barbarian should get this final benefit as a Bbn1, coming as it does after a series of higher-level abilities. Fortunately, the city brawler description says "Level: 1st" and the only level-associated entries are otherwise specific to 6th and 11th, so that should be a nonstarter. But it's still a poor layout.
Finally, the spell master's touch [div] (SpC 139-40) grants "proficiency with a single weapon... you hold in your hands when the spell is cast." As it's a 1st-level spell even a Cha 11 sorcerer can cast it; as its components are pretty much just vocal (the weapon you're holding's the focus) Arcane Spell Failure isn't an issue; and as its casting time is a swift action that you probably weren't using anyway now you've something to do with it. The Player's Handbook calls out nonproficiency as the reason for the -4 penalty when using improvised weapons (113), so the spell master's touch totally grants the caster weapon proficiency with that table. Improvised weapons, however, don't appear on Table 7-5: Weapons (116), so there might be some resistance.
Taking 2 feats--the aforementioned feat Improvised Weapon Proficiency and the homebrew feat Improved Improvised Weapon Proficiency that eliminates the remaining -2 penalty seems more than reasonable except in games where either the DM is hilariously tightfisted and your PCs are scrounging for the next copper piece or the DM's campaign is some kind of weird rust monster apocalypse.