Are we talking FW
liveaquaria.com has a great reputation and I have used them, though they are pretty pricey.
Some of the best sources are hobbyists who breed fish. For FW
fish, check out msjinkzd (google) or Private message HN1 a/o Severum Momma here on the site - they can usually get just about any FW
fish, if they don't already have them available.