forums are a great place to buy fish online, usually alot cheaper than websites devoted to online fish sales. If you check out the classifieds section on here, you'll see a few threads from Severum Mama... I've heard nothing but great things from people who have purchased fish from them, and they're good people too.
I have no first hand experience with any of the websites, but if you search this forum for info, there have been tons of threads talking about the various retailers (good and bad).