Make your own medicine ball with a little help from a power tool. Use an awl or a drill to put a small hole in one of the black stripes on a basketball. Make sure it's big enough for the hole of a funnel. Funnel in some sand, fill it up to your desired amount, and then patch it with a tire-patch kit or, if you don't have those resources, cover with a few layers of heavy-duty duct tape. Voilà!