I've been out of it for long enough that I don't know where the "in" place for ordering is these days. Where do you like? None of my local fish stores carry Tanganyikans. (The biggest one has a lovely selection of South American cichlids, though.)
look for local breeders in your area or you could find a fish club, there are a lot in Pheonix and i saw one in tucsan on the internet. also lfs might have stuff. What kind of fish are you looking for and are you willing to drive in the california area, because i know some good breeders here that have pretty much every tang and they are cheep.
I probably should have specified "mail order", I'm sorry. My LFS don't carry Tanganyikans, I have yet to find any fish enthusiasts in my town that do Tanganyikans, and oddly enough, given gas prices, it costs almost as much to ship them down here as it does to drive to Phoenix. (Although that's my next job, to start calling fish stores in Phoenix.)