Look for used tennis ball machines on eBay and Amazon.com as of 2015. These sites also offer accessories for tennis ball machines, such as wireless remote controls or storage covers.
Due to the rapid output, a tennis ball machine puts a lot of wear on tennis balls, especially with frequent use. To maximize the lifespan of the tennis balls, it's recommended to load the tennis ball machine with pressureless balls. A pressureless tennis ball is one that has a solid rubber core, while a pressurized ball has a hollow core filled with nitrogen or pressurized air. Pressureless tennis balls are more durable than pressurized tennis balls and are often sold in larger quantities than pressurized balls.