I'd say your best bets would be Craigslist, Ebay, GMforum.com or Pontiacbonnevilleclub.com. Watch out on PBC as it's kinda known to be Pissy Biatch Club. The admin there is a douche waffle of epic proportion.

They would problably have the best need for it though. And IIRC there's a guy in your area that still has a S1 Bonneville that he replaced the motor with and might need a spare.