I always buy my tires at Discount Tire on Hampden just East of I-25 because they are a pretty good deal then they give me free rotation and balance for the life of the tires. They'll try to sell add ons, but you don't need to pay for them. They take care of you regardless. Costco or Walmart may be a little cheaper, but I don't know what their policy on service is.
Oh yeah, I have no idea what kind of tires to buy. I just give 'em a price range. Last time they put Pirellis on for about $75 per tire.