I second getting on the phone and calling people. Try the following:
-Laundry Mats
-Carwashes (found a working Dr. Mario at a car wash but he won't sell

)
-Antique shops
-Churchs (found my Space Invaders at a church)
-Restaurants ( mainly Ma & Pa places that don't rent their machines)
-Skating Rinks (found a Wacko machine here in Panama City if anybody wants it

)
-Pawn Shops
-Arcades (call to find out if they sell broken or empty cabs, if not find out the name of the distributor that delivers to them the cabs and call the distributor)
-Gas Stations (don't call, just notice if they have any or not, there is a gas station in my town with a working Killer Instinct 2 or something like that. It needs a cap kit BAD!)
Thats all I can think of. I am sure more people will suggest places.
Always ask for the manager first, then ask if they have broken, or non-profit making machines for sell. Most of the time they assume you are wanting a working machine and fail to mention the non working ones.
Welcome aboard!!!