Mylar. The term Mylar is often incorrectly used. It is a specific type of platic that is heat resistant. If you are using it for shapes then you just need a platic and not the no melt. Mylar is probably too stiff and hard for cutting stencils if you are using a cutting knife but is suitable for the buring tool. Teachers often ask for Mylar in their supply lists when they mean "stiff plastic". It is easy to cut with scissors but harder with knife.
If you have the double blade knife then it i easier with a more flexible plastic such as DBK which is blue and flexible.