I made stencils a while back from something similar to acetate, it was actually film used transparent blueprints. Kind of similar to xray film, I got mine used from a civil engineering firm. Stencils came out great, and a nice benefit of them being transparent was that after the first color was dry, it was easier to line up the stencils for the second color. They can be re-used too, I cleaned the paint off after I was done, and gave them away on RGP.