I was just thinking....the Bender Ball is just a small, hard ball right?
They sell those at Target and Toys 'r' Us! They have millions of small hard balls in those cage things! I have one downstairs and I'm going to try doing some sit ups or something on it to see if it does anything.
It's also entirely possible that I might be looney and maybe it won't work at all...but anything to save a couple bucks.
EDIT:
Just checked the website - it's a soft ball. But I'm sure they sell those too! It looks like something you could find in a toy store somewhere!