Well not having any clue what type of area you're in... Keep your eyes open, if you see one ride by, hollar at the guy. Go ask around at motorcycle shops. Ask at bicycle shops. Make sure to ask at fleamarkets. If you or someone you know is an alright mechanic, check the junkyards. Most likely, depending on where you are of course, you may end up having to simply order one from somewhere. I'd recommend if you do that, look on www.moped.org at the sell listings, or search ebay for mopeds.
swarm and destroy