Recommendations for a floppy heart prop?

I'm working on a life-sized beating heart, and I want it to look as realistic as possible. Making it move won't be too difficult (servos), but I'm having a bit of trouble figuring out how to procure the heart itself. Real beating hearts are very squishy and floppy, but everything I have found is made of cheap, semi-flexible plastic.

So, does anyone have any ideas? If necessary I can buy a heart-shaped jello mold and fill it with latex, but I have no experience with such an involved process, nor do I know if that will create exactly what I want.