Some of the best
Jeff Rapp at Tangled up in Ciclids is excellent for New World except for Apistos which he doesn't really stock (but Chris Moscarell up in Spring Harbor, PA breeds lots of them and is a CCA member), Redkahuna on Aquabid (in Baltimore) is a great breeder/source for Corydoras, Atlantis Tropical Fish Hatchery in NY is very good for Old World. All sites can be Googled for purposes of contact. CCA monthly meeting auctions if you haven't yet been are generally exceptional as well and you won't find better deals on healthy and interesting species anywhere near the DC/Baltimore area. You could also post a wish list under the Marketplace section and see what happens.
Luck.