I have been using the OfficeMax Clear Front. They don’t sell it online, but use it in the stores to front presentations when they bind them. $0.29 a sheet. You might need to go to a larger store and then convince the person that you need it and that not many people will be coming in to bind presentations.
Albertozok, you are incorrect and also no one is forcing you to use stl files provided or linked off this site.
This guy shows steps necessary to make a custom fit mask. It takes time to design but once you generate a file for a person, they can print as many copies as they want for themselves.
Also, a more universal approach is to start printing in PLA and then switch to Ninjatek or similar flexible material. Cura supports this feature.
I left out the link to custom mask design - https://www.youtube.com/watch?v=kBQcNBPRkh0