It is release paper from Amazon. I forget exactly whoch size I bought, but here's an example: https://www.amazon.com/dp/B08FQSY87M/ref=cm_sw_r_cp_apan_glt_i_TMW5XRSM0421KT32J167 it is reusable and works very well. The ones I have on this painting are a little beat up because it fell behind my couch for a night by accident.
I use release papers that I found on Amazon and get rid of the plastic sheet. I add stickers to the sheets to make me smile.
Here’s the release papers. 200 Pieces Diamond Painting... https://www.amazon.com/dp/B08FQSY87M?ref=ppx_pop_mob_ap_share
I also have cut the clear cover with an exacto knife but I can’t cut straight to save my soul AND I’ve gotten the wrong side down on the canvas before.
I believe it is called release paper and you can buy it on amazon. Ex: https://www.amazon.com/Painting-Replacement-Double-Sided-Non-Stick-Accessories/dp/B08FQSY87M/ref=sr_1_1?crid=1K5B3U8YXRPDY&dchild=1&keywords=diamond+painting+release+paper&qid=1614260981&sprefix=diamond+painting+relea%2Caps%2C184&sr=8-1
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I believe you can also use parchment paper (NOT WAX PAPER).