Sold my M Roadster and bought myself an E28 M5, so I have a Sprint Booster leftover that I don't need.
Best decision I ever made was putting this on my Z4M. I highly recommend it (even if you'd prefer to buy new instead of buying mine). It's 10-months old and I have the original box. Part number is SBBM0002S.
They're $300 new at BavAuto. I'll sell mine (shipped, USA) for $150. If you live in LA and want to come pick it up from me in person I'll be happy to knock off $10 and sell for $140.
Note that I DO NOT know if this is usable on other models (and if it is, which ones), so if you have something other than a Z4M you'll want to do some research.
PM me if you're interested or have any questions.