*WARNING* Not all HEPA filters are safe and MANY utilize fiberglass! 3M filters do not contain fiberglass, they are a poly plastic, so these are relatively safe, but if you are not 100% sure AVOID HEPA filters
Blue shop towels are the most easily available and cheapest option.
Polypropylene interfacing is a highly recommended material for masks. Be sure to get the right kind, as not all are made with the optimal filter pattern. Please see the study below.