GoFit sells stability ball plugs. The plugs are designed specifically for GoFit's balls, but if you have a BalanceBall (made by Gaiam) or a stability ball from another manufacturer, it might be worth giving these a shot.

http://secure.gofit.net/view_product.php?product=17580

The shipping cost is more than the plugs ($4.95 vs. $2), but the total cost is still less than a new ball.

And yes, we found this out because we lost the plugs to our stability ball. :)