If you are into a little "do-it-yourself" work, I have noticed hollow plastic golfballs in the toy sections of stores this summer, usually with a pair of golf clubs attached. Even the Dollar store has had them. Another source seems to be pet toys sections- they show up there frequently. You could drill a small hole, insert the (mmmm-mmmm I can't say it!) and then fill up the rest of the ball with modeling compound or play dough if you need a little weight to it.