Where can I find a source of used 50cc and 90cc scooters that I can refurbish, repair and re-sell?

Check the buy and sell on line.

also classified ads in the paper and site like craiglist, ebay etc.
you may be surprised what you come up with.

craigslist > for sale > motorcycles

Trucking companies might be a resource for freight damaged scooters.

Check local auctions in your area. Place your own adds in newspapers wanting to buy scooters, put advertisements up at any universities or colleges nearby.