Bimmerworld has Continental Challenge (rebranded Hoosier R7) take-offs with 2-4 heat cycles for sale. Got a set of four 225/45-17 for under $400 shipped.
Update: Wow, the 25% off deal seems to have closed already, but they still have the same tires for $100 each, so should come out to just under $500 shipped in the US. The sales rep said they came off the cars that raced at Sebring.
http://store.bimmerworld.com/clearance--used-c21.aspx