New or used?
My friends look in the dumpster of small tire shops, and steal them at night after a drift event.

New: Buy them online. Tirerack.com and Discounttiredirect.com always have the best price. I prefer to use both. tirerack charges shipping ($5 per tire) more, while discount shipping is included in the price.

Used: Ive used AKA Tires on ulmerton near the airport, next door to Enhanced Concepts. They usually have 205-225 in stock off of bimmers. they charge $30 per a tire. If you spend $30 more per a tire, you can get brand new tires.

If you buy them online, and need them mounted and balanced, i have a friend on Haines road that does them for like $10 per a tire, while everyone else is charging $12. I think its called Danh Tires. They also have used tires too, so might want to check what he stock up on.