I've heard people around multiple sites recommend Aquarium Fish: Tropical Freshwater Fish and Saltwater Fish for Home Aquariums
. However, I believe it's best to buy a lot of fish at once, to offset the high shipping costs. I do think they offer exceptional service though. I've read through their site wondering how they ship fish and everything, and it seems like they take all of the necessary (and unnecessary) precautions to keep your fish alive. They also have a 14 day guarantee for fish and a 30 day guarantee for corals.