Tong's in Fountain Valley is hands-down my favorite place to shop. They can be a bit expensive, or great deal, depends on what they have and what you're shopping for.
405 in Westminster is probably the cheapest shop around, but has a limited selection, and their livestock is not always the healthiest. I've had great luck with their shrimp, not so much with their otos.
If you're a SCAPE member, and/or pay in cash, either place is usually willing to bargain.