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I've heard only good things about Drs Foster and Smith--although they are expensive. I have not used their fish shop myself though.
 
I like vividaquarium.com and liveaquaria.com
 
I have only heard great things about liveaquaria.com and would simply add the note that if you are purchasing "special handling" fish you are aware of it before you order. Typically they will state that the arrive live guarantee doesn't apply. This is a small portion of what is shippable, but worth noting nonetheless.
 
I try to check out the classifieds here before I shop. I like dealing with other users. :)
 
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