You'll have to google the addresses:
Little Amazon on Aurora Avenue. (Pretty decent discus and flower horns, random fish too.)
"The fish store" on Roosevelt way in Seattle (SW and freshwater, mostly small fishes, wild discus, pretty pricey)
Aquarist World in Seattle/White Center (Was there once, had a pond of huge pimas for display. They have random fishes, Flower horns, rays, not the best variation of fishes, but A LOT of tanks.)
A Place for pets in Burien (Pretty well stocked with fish every time I go, Saltwater and FW)
Midway Tropical Fish in Kent (Good selection of fish, haven't been there for a while)
Serria fish and pets in Renton (Decent selections, Saltwater, but mostly smaller FW fish and plants)
China Town:
QT Aquarium (Flower horns, Saltwater, Discus, Tigs, RTC, etc)
King Discus (A few Discus, random fishes)
Aquarium Plus (plants, shrimp and smaller fish/koi)