TireRack or Discount Tire is where I've always gotten mine. I've found Discount to be easier as they will match TireRack's price and dn't charge you extra money like some places for installing tires they did not sell you.
Price them out on tirerack and discount tire, print it out. There are many local type tire places in town that will beat competition prices by 10%. Then take that quote to another shops that does the same. I've made out like a bandit doing this. I've been able to pull 30% cheaper than online doing this trick.