I didn't describe it properly. This is the low-adhesive plastic film that comes on products from the store, and that you remove before use. When you buy a digital voltmeter, there's usually a plastic film on the display window with fake letters, and you peel it off before use. On my printer, there's a smoked gray plastic cover that folds down over the paper feed when it's in use. When I bought it, the cover had a thin film of plastic on it to protect from scratches.
In my case, I have a rectangular plastic cavity cover on a guitar that still has the protective plastic film on it. The edges of the film have started to curl from handling, and I'm selling the guitar as new (which it is), so I want to replace the plastic. I just need a little bit.
I have a pick guard cover that still has the plastic on it, but there's not a big enough area for me to use.