I've purchased a _lot_ of tires (both street and track)
I've used TireRack.com, DiscountTireOnline.com, 1010Tire.com, multiple companies who sell aftermarket parts (some who do full builds/tuning), walked into B&Ms like Discount Tire, Tire Kingdom, etc.
It really just depends. My last set was a semi hard to find setup (285/35-20 front, 305/35-20 rear), I wound up getting a nice set of Nitto NT555 G2 for a good $300 less than the next best online price, $400 less vs. local B&M, from an online store that sells aftermarket parts for modern domestic performance cars. There's a killer small tire shop in town, excellent work, exceptional prices, did an M&B (no disposal I took the NIB wheels directly up), for under $80
It really just depends. My last set was a semi hard to find setup (285/35-20 front, 305/35-20 rear), I wound up getting a nice set of Nitto NT555 G2 for a good $300 less than the next best online price, $400 less vs. local B&M, from an online store that sells aftermarket parts for modern domestic performance cars. There's a killer small tire shop in town, excellent work, exceptional prices, did an M&B (no disposal I took the NIB wheels directly up), for under $80