Free, thin-like-stiff-paper, flexible 16x20 transparent plastic sheets. We have hundreds of them. Not sure what they are made of -- maybe mylar? (No idea -- not a plastic expert!) They are smooth and shiny and can easily be cut with regular adult scissors. (We use them for making stencils, but we have way too many of them.)
When we buy canvasses for our art studio, they are on the surface for protection under the plastic wrapping. We hate to throw them away, since they could be used for art projects. Cover with tissue paper shapes for "stained glass", use for tracing stencils, placemats, paint palettes, Cricut machine shapes, etc.?
Our studio is open during our 2 hour classes, and during set-up for each class. Just let the teacher know when you get there why you are there and he/she will hand you a stack of them. (Please do your best to not interrupt the class any more than necessary to pick them up.) We're basically open every evening from 5:30pm- 8:30pm, 11am-9pm on Saturdays, and Sunday times can vary.
Class schedule is here: https://mastersitedesign.com/painthampden/
Our address: 6461 E. Hampden Ave, Denver, CO 80222. We are behind the Conoco station near Monaco and Hampden, just east of I25.