Coroplast and Sintra both cut nicely with a utility knife, wall cutter, or mat cutter. For embedding object mounts, you can easily peel the skin of Coroplast by slicing with a #11 X-Acto knife.
Both of these plastic sheeting products are commn to the sign-making industry. So, a wholesale supplier to sign makers would have them in a variety of colors and thicknesses. Industrial distributors of plastic sheets & tubes may have them, as well. Shop around your metro area.