I found a great deal on eBay today. This deal is from a non-profit organization who sells used tennis balls to support its mission.
RecycleBalls, the seller of the used balls, sells tennis balls in 3 different grades. Grade A to grade C. Grade A tennis balls are all from indoor courts. Grade B balls come from both indoor and outdoor courts. Lastly, grade C comes from outdoor and kids courts.
You can purchase 100 to 400 tennis balls in bulk. The seller has sold over 690 sets and a highly rated seller. Here is the link to purchase the used tennis balls from RecycleBalls.