I bought some from amazon before, they were high quality, delivered in a hard paper envelope to keep them flat. The PTFE sheets I tried were kind of annoying to work with since any hair or dust clings to it. They can still tear or fold too. Depending on how many sheets you’re actually going through, check suppliers of bulk.
There is option of, silicone or quilon which apparently is mostly comprised of isopropyl alcohol so probably should not be used?