Vinyl
I purchased mine from eBay, look up seller rcpromask. I just picked out a few and pieced them together to make it a little more one off. I've shopped around for vinyl rolls and it's pricy so I went the liquid mask route, if you want to play with vinyl and not blow your paint budget on mask material I would look up local sign shops in your area and see if they'll sell you remanence from their stock.