I bought a blank paper mask in a $1 store, and then painted it. The paint design is my own.
The paper is maybe 1.5 times as thick as a cereal box, so it's fairly flexible, but still thick enough to withstand being varnished. It's only varnished on the front, as the varnish makes the mask much stronger, and tougher, but it's harsh on the skin, so the the back is plain paper, and is soft against the skin.
The one disadvantage of the varnish is that it has a yellowish tint and slightly discoloured the blue and green so it's not quite as nice as it was before varnishing, but it is now stronger, and splash resistant.