Another good simulator, but one not specifically designed with breasts in mind, is OE-Cake. While it's more of a catch-all sandbox physics simulator you can import any drawing that's saved in .PNG format and it'll become an object made out of whatever material you designate, including squishy, bouncy elastic.
It'd take some messing-around with, but you could certainly pull it off.
http://www.octaveengine.com/en/casual/trial.html
ALSO: You'll want to make sure the .PNG image has a blank background and not a white one, I know you can blank a background using Macromedia Flash, not sure what other programs do it, probably anything like Photoshop would work.