I got one from Wal-Mart, its the 'Gold's Gym' brand. I think it was like 18 bucks, but it had sand in the bottom to counter weight and make it more difficult to roll off of it. (They also had some that were literally just exercise balls for around 10 dollars in the same section.)
Reebok brand from Target with a weighted bottom. I really like the weighted bottom feature, it helps keep the ball from "getting away from you" when you move around on it.
Re: Recommendation on Birth ball/balance ball
I really like the weighted bottom feature, it helps keep the ball from "getting away from you" when you move around on it.