eBay usually has them, use the apple serial number checker to make sure of the model then find the part. Google shopping works too. and I'm sure ifixit sells them as well. For your case I've done it for you and it's 661-5101 for the 2.0 GHz one and 661-4819 for the 2.4 GHz one. Good luck and don't forget the thermals.
Great! Was able to replace my wife’s engine which started to be very buzzy for some reason.