The optional replacement would be "self-adhesive laminating sheets". These are used for protecting photographs and documents. They are thin clear sheets with adhesive on one side. You can find them at most office supply or arts and crafts stores. In the 9x12 size there is the Avery brand, product number 467142. [link]
I had the same problem a few years back when I wanted a replacement pen for an old tablet, that was out of stock. I emailed them, and they adviced me another pen that wasn't originally intended for the tablet, but did work on it. Maybe they have some good advice for you as well?