Casein, a protein found in milk, can be easily precipitated from standard moo juice with vinegar and turned into a malleable homemade plastic. Coffeebot wrote an Instructable that shows you how:
The final product is quite rigid when it’s thick (1/8 inch or thicker), moderately pliable when it’s a little thinner, and brittle if it’s paper thin. It’s also sandable and paintable.
I guess casein-based plastics used to be all the rage for buttons, jewelry and pens at the beginning of the 20th century.
See also: Moldable plastic from styrofoam