Hello friend!
If you need a filter for your mask:

Measure the mask from chin to nose:

Download the filter shape pattern and cut your filter material to shape according to your measurements.
Filter material can be obtained from HVAC air filters, vacuum bags, or surgical sterile equipment wrap (Serfinity Medical, USA Medical & Surgical).
If using 3M air filters, get ones with at least 1500 MPR rating (source).
Air filter material can also be purchased in bulk from Filti.