Depends largely on what you want. Special teas is nice for decent tea. as is Adagio.
If you want to go more high end,
http://funalliance.com/ is a nice start.
My favorite of all times is
http://www.jingteashop.com/default.cfm (Great tea, enthusiast level, direct from china).
Also, Scott at Yunnan Sourcing if you get into pu-erh like me
http://stores.shop.ebay.com/Yunnan-S...__W0QQ_armrsZ1