Think you can only get one of those from an Apple Authorized Dealer... not sure if you have to have a defective exchange for it... also, if Apple parts pricing is what it used to be like, I think the price on it would make you just want to buy the whole computer!
You might want to try d'Bay... but single part that new would be hard to find. Did find a singleprocessor one going for $365 with a 5-6 hours to go...