ebay is the cheapest usually; however, being very cautious is a good thing:
check out the seller's feedback, and I prefer buying it only if it is advertised as new and in an unopened factory sealed box.
There have been many threads here regarding this issue - do some sellers email a previously used registration code? Some have reported that Apple has refused some activations; in some cases you won't know until you'd actually need to take advantage of the Applecare because of some problem. You can avoid that by only buying it in a factory sealed box.
You can find the place in LA here (I've purchased from them as well):
http://www.lacomputercompany.com/cgi-bin/rpcart/index.cgi?command=listitems&type =group&group=appcare