I have seen the melanistic Gambusia males several miles south of here, they were not common but if you were persistent you could net a few. Trouble is the females are drab and Gambusia are mean, nasty little fish, which I no longer keep, even in an outdoor pond. Used to keep them with African Cichlids if that is any indication of their nature and they held their own for months. There is a melanistic form of the Golden Top Minnow that is quite striking, but I have never caught one.Sachs aquaculture and Jonahs aquarium both sell melanistic gambusia (mosquitofish) whenever they have them in stock.....really cool looking little predators.