Right now at Amazon you can get a pack of 50 Disposable 3 Layer Anti-Dust Protective Cover Face Masks for just $16.90 plus free shipping with your Prime membership. These are adult size face masks with a three-layer cloth filter design.

I am actually ordering these again for my husband who travels work and likes to have these just kept in his car at all times.

Amazon 50 Pack Disposable Face Masks