Sorry, I should have been a little more specific. My hardware store sells plexiglass sheets, so that's what I use, but I believe any kind of thin plastic sheet works. I haven't seen it at Home Depot, and I rarely shop at Lowes, I don't know about Harbor Freight. Maybe like an Ace Hardware if there is one near you. Do you have any other store, maybe like a privately run one? The one I buy it from is a private business.
Other than that, maybe try ordering some online.