Another vote for TireRack.com. I have been buying tires there for years. Always the cheapest, always the quickest delivery.

I just looked up tire prices yesterday at 3 of the larger sites; Tire Rack, Tires Easy, and Discount Tire Direct.

I was pricing a full set of Goodyear Eagle F1 All Seasons tires.
Goodyear Eagle F1 All Season 255/35 ZR19 96Y XL VSB
Goodyear Eagle F1 All Season 225/40 ZR18 92Y VSB

Prices including shipping --
TireRack.com: $749.78
Tires-Easy.com: $888.83
DiscountTireDirect.com: $855