My favorite is http://www.fosterandsmithaquatics.com/
They have all kinds of fish supplies as well as a small selection of aquariums. If you're looking to buy a home for a betta, the Marineland Eclipse tanks are a reasonable choice. They come with a good fluorescent light and a power filter that's easily baffled, and are extremely quiet. You will have to buy a heater, though. I suggest the adjustable 25Watt hydor theo or similar. The prices there are very fair, often $10-20 cheaper than what you would find in a pet store, and the shipping is $9 flat rate no matter how much or how heavy the items are.