I have a 3 pound ball I got from Walmart. The box says its a medicine ball. They also have 5 pound balls. These look to be filled with sand.
I LOVE mine. It came with a little fitness booklet. Has like 12 exercises you can do with it. The cost was, if I remember correctly, $7.87 for a 3 pound, and $14.87 for a 5 pound.
Dunham's Sports also has these for about the same price I think. I buy my weights there now. They are lower priced, or similarly priced, to other stores around here, and are close by.
Anyways, I really love it. I am wanting to purchase another one.