Get a basketball(4$ @ walmart)
Bag of sand(3$ or free if you are near a beach or park)
small funnel 1-2$ ( harbor freight or other hardware store)
Tire plug kit ( maybe 5$ for 3-5 plugs)
1-Punch the valve out on the ball.
2-push the funnel in the hole.
3- Fill with sand.
4-plug the hold with the tire plug.
I have a smaller basketball at 20lbs, and a full size at 30lbs. They are holding up great. Harbor Freight also ans a good sled strap for cheap.
Or if you want to buy one, the "Slammer ball" is good and a comparatively low priced option.