Ebay, but be aware of the scammers. If something doesn't seem on the level then avoid that particular purchase.
Wreckers should have a engine for you and it may have some sort of warranty if you find the right salvage yard.
Someone on this site or another bike site may have an engine for you. Check out the Bike/parts for sale section, heres a current thread with a motor http://www.r1-forum.com/forums/showthread.php?threadid=62356 or you could go to the Bike/parts wanted and put your request in there.
I have a 100% bone stock motor I'm selling. I was going to put it on ebay in the next couple of days, but if you want to make me an offer on it, feel free to PM or email me.
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