The only online seller I use for corals is AquariumDepot.com. Everything else named in this thread is very much overpriced in my opinion.
e.g.
http://www.liveaquaria.com/product/p...72&pcatid=1972
Thats a 5$ coral for 50-70$...
For fish and other inverts, bluezooaquatics and saltwaterfish.com.