(Diarrhea-post for the sake of increasing forum activity)
Either tennis balls or racquet balls seem the safest bet, as far as easily-obtained and safe ammo goes. I'd recommend at least wearing a cup and something like racquet-ball goggles for protection. I'd probably just use my fencing mask for face protection, even though it's a bit overkill.

Perhaps short-ish slings would also help a bit? More like a guideline than a requirement, since some would probably like to plink away from afar, and some probably just can't hit worth a da*n with a short sling