Most sewing machine / vacuum cleaner repair shops should be able to sell you a used one cheap!
I got mine for $10 from a place just down the street from where I work.
Go to
www.superpages.com, search for "sewing machine" and you should see a category for sewing machine parts and repair. Then just get on the phone!
I used Greg's suggestion to tell them I'm working on a project and just need a working motor. The guy I called had three for me to choose from.