There's a place called Fish Gallery and Pets in Renton that I go to a lot. Their fish are in pretty good shape, and they have a very wide selection, especially of Rift Lake and CA cichlids.
For certain selected species, however, I go to a place called B&D Aquarium in White Center (a neighborhood in West Seattle). The shop is small and selection is somewhat limited, but they are very rigorous about quarantining their fish (it's the ONLY store I'll buy cardinal and rummynose tetras from, for example). They also get occasional shipments of relatively rare fish, such as Semaprochilodus taeniurus and some interesting plecos as well. Oh, and if you like Irianjaya rainbows, he always has a beautiful selection on hand. For all this, however, he is not cheap. Still, well worth a visit.
Hope this helps.
