I've just had to go through this process of deciding to buy locally, or buy online, at either TireRack, or Discount Tire Direct.
3 sets of snow tires were involved, for 3 different vehicles, in 3 different sizes.
My RX-8 uses 225/45-18 tires, and are not cheap, by any stretch of the imagination. My gf's Fiat 500 is brand new, uses 195/45-16 tires, and are pretty much impossible to find a snow tire in this size at all. Her PT Cruiser uses 195/65-15, and is the easiest and cheapest of the vehicles, and made life much easier that way, but sometimes too many choices can give you a headache.
Anyway, for the RX-8, I compared online venues only. TireRack vs Discount Tire, and Discount Tire Direct. Prices were comparable, but TR adds in shipping and tax, DT has only a $30 rebate Visa card, and DTD has a $30 plus a $40 rebate Visa card. So, DTD won out, as the overall cost was less than TR, and far below the cost of the local DT store. The $70 worth of Visa cards pretty much pays for installation.
The Fiat, on the other hand, was hard to find a tire for. We had to switch up to a 195/50-16 tire to get some good choices. Eventually, we settled on the tires offered by DTD, as the same $70 worth of Visa cards makes everything more economical. We're doing the same thing with her PT Cruiser tires.
So, shopping for tires at DTD saves us huge amounts of money on taxes, shipping, and with $210 in rebate cards, even gives us free install.
BC.