On Walmart.com where you can now get this
36 Penn Championship Extra Duty Tennis Balls for only $19.99 (regularly $34.99) – just 56¢ each!

These tennis balls are used by professionals everywhere, even in the USTA League Tennis matches. The natural rubber gives these sports balls a consistent feel and reduced shock. You’ll get 12 cans with three balls each for a total count of 36.