I've got a few books that have prestige classes for Bards, but I'm just going to sum up my favorite. These are from Path of Magic by Fantasy Flight Games. If there is a certain element of the Bard yoiu are trying to findf a prestige class for please post that, I might be able to come up with something. There are too many books to just list everything, but usually with a more specific concept I can come up with something.
Chanter: The legendary dwarven bards who use drums and timed beats to drive people to work harder, better, and faster
Grand Diva: The ultimate in social singer, she thrills audiences and masters the art of singing.
Jester: The altimate in clowns, the Jerster is booth funny and deadly.
Spellbinder: THis is more intended for an Enchanter but bards can qualify.
There are also Legendary classes in there. They are like prestige classes, but one has to be at least 12th level and they are much harder to qualify for.