I have an idea that a few friends of mine suggested to me and I tried and it works. ^_^;; Instead of using balloons, use pantyhose!
Basically, you get a pair of pantyhose, and cut off the legs.. then you fill each leg however big you would like to be with either rice or birdseed, and then tie it, wrap the end around the entire thing again, and tie it one more time.
^^; The result? beanbag like things that have a good weight to them and conform to a shape. Plus, it's pretty hard to break them because pantyhose double-layered is pretty strong and hard to rip.
The only thing I haven't figured out yet is how to keep them IN the bras... ^_^; They sit in there pretty well, but unless they're secure they are prone to falling out... so basically wear a tight top or a sports bra! Hehehe.