I've ordered fish from two different places;
Liveaquaria.com
Adequate service, so-so prices. I got a pleco and some corys here that arrived in good condition. The tetras I got were also decent. However the last order of two angelfish was not ideal. They looked like they had been dropped in a garbage disposal for a second or two - fins ripped to shreds. The plants however have not been very good, a bit scrawny.
AZgardens.com
Very good service, decent prices (except on shrimp - I'd try franksaquarium.com). The fish I've gotten from here have been great, though selection is more limited they have some very interesting choices. Their plants have been very good, very healthy.