The Black Shark in Lewiston is worth checking out. I went there a few months ago and they had a fair number of unusual fish like celestial pearl danios, discus, various gouramis and cats that you don't see at the stores around Portland.
Agreed. An excellent store.
More locally the best I've found so far is the Pet Quarters by the mall. They have a lot of tanks and more gourami and cichlids than you'd expect from a non-specialty store. Prices on livestock are quite good. The Bad: The markup on equipment etc is ridiculous, and fish health can be very hit or miss. Both standard for big generic pet stores, but that isn't really any excuse.