anything custom you'll need to go to an audio shop (NOT circuit city or best buy & the like). around here there are shops like House of Sounds, R&D audio, ZM audio, etc...you get the point.
most custom shops are locally owned, stand-alone shops and not chain stores..so dig out your yellow pages and look around
it's also a good idea to know what you want before going in there..they'll be able to offer you advice as well