obviously craigslist for used tires....but new?
http://www.discounttiredirect.com/direct/home.do (tends to have really good discounts)
http://www.discounttire.com/dtcs/home.do
http://www.4wheelparts.com/ (free shipping if picked up at a 4wp store, decent deals)
http://www.summitracing.com/ (cheap shipping to your door, decent deals)