I wanted a more organic, fleshy texture of my air bladder, so I cast it out of liquid latex. Liquid latex of many different sorts can be bought in a craft store, prop shop or easily on the internet. I mixed the latex with red pigment to color it, and painted it, in layers, onto the outside of a large balloon. The many layers built up to form a big, floppy fleshy balloon, with the texture I created with the brush.
A simple balloon, beach ball or even a garbage bag could replace. Check out this website
for different types of large-sized balloons.