You can pick them up at pretty good prices. Try looking at swap meets too. DIY Generator F or all of you DIY fans, here is an easy project to occupy some of your spare time. Find an old gas powered lawnmower. Yard and garage sales are good places to find these.
Many people sell their old lawnmowers for a few dollars when they upgrade to a newer one.
Here is how you can take an old lawnmower and make a DIY Generator. This pulley should have at least a 6 inch diameter, to increase the mechanical advantage. Find an old 12 volt battery and car alternator. Remove the pulley from the shaft. Cut a hole in the deck of the lawnmower that is large enough to put the alternator drive shaft through.