Liveaquaria.com seems to be one of the most popular choices. I, personally, have boycotted Marine Depot Live. Although I love their dry goods, their live fish department demonstrated (in my opinion) some gross incompetence when I ordered from them. I've also ordered from phishybusiness.com. I'm holding off judgement on them. Out of an order of 5 fish, one was DOA, and another died a few days later. This may very well have been a fluke, though. Their customer service was very good (refunds with no questions asked), and they do at least appear to have genuine concern for the health of their livestock.