Jo Jo Aquarium in Sacramento sells gulper cats. I haven't been to check it out myself, I live in the east bay and its a bit of a trek for me.
6th Avenue Aquarium in San Fransisco has a lot of cool fish, I see stuff there I never thought I'd have the chance to see. One thing, they're quality is baaad, be sure to treat anything you buy.
Concord Aquarium in Concord has a decent selection, but it is a tiny shop. Steve the owner is a great guy, he'll order fish for you, and he gets in neat stuff from time to time. He's got a very pretty Royal Clown Knife right now.