I have used tirerack.com many times and recently treaddepot.com
The key to buying tires online is two-fold.
1. They run specials where shipping is included on certain brands of tires
2. Often they have prearranged deals on mounting so there are no surprises.
Savings can vary from significant ($30 per tire) to nominal and not worth the hassle. Free shipping and a savings of about $20 per tire (times 7 since I was replacing my spare as well) was worth the effort.