I've been buying my tires from them for about 3 years now, and although their prices aren't the absolute rock-bottom cheapest, their customer service more than makes up for it.

They are located in Kentucky and I usually have tires within 2 days, sometimes as quickly as the next day (all orders over $200 shipped free). They are courteous, professional and a pleasure to do business with. In addition to tires, they have a wide inventory of the usual apparel and bike accessories. I've NEVER had a bad experience with them. If you're loccated east of the Mississippi, you might want to give them a try.

http://www.ridedirect.com