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If you need tennis balls for purposes other than playing tennis (for your dog, to put in the dryer, to put on the legs of furniture, put on a walker, etc.) you can sometimes find old tennis balls by scouting your local tennis courts. Frequently tennis players will be playing with lots of balls and may overlook a few around the edges of the court when they pick them up. Also sometimes if a ball is too old for them to play with (has lost its bounce, has a crack, etc.) they will just leave it at the court.
Source: Personal experience, former tennis player
By TheFrugalTownie from Lawrence, KS