I was in the grocery store the other day, and these balls were being sold in the pet section. They have approximately the same weight as a regular tennis ball, but they are less than half the size (1.8" diameter). I like them better than regular tennis balls. They are tougher, they appear to fly somewhat faster and they are more similar in size to the stones that I sling. They're also sold on Amazon.

Good find. Sometimes the pet section has interesting projectiles.