You can buy the plastic window protectors at Tractor Supply and save shipping UNLESS you have the BIG window like mine and in that case I have to buy them from TPTool since tractor supply don't carry the big ones. I am actually going to see if I can find it at a craft store. I know its out there somewhere.
Ask for clear acetate. A real art supply store should have it.
HERE'S what you're looking for and if you can't find it locally, [censored] Blick has the best prices anywhere on artist supplies.