Some examples of supply available include:
https://www.amazon.com/s?k=n99
https://www.ebay.com/itm/3M-6300-Large-Half-Face-New-Reusable-Respirator/324062405641?epid=26014084129
https://www.ebay.com/itm/3M-7093-Particulate-Filter-Fits-5000-6000-7000-Series-NIOSH-P100-2024/133362247876
https://www.amazon.com/Particulate-Filter-Compatible-FF-402-Retainer/dp/B085S3Y8BP/
https://www.amazon.com/Activated-Filters-Exhaust-Replacement-Neoprene/dp/B085C68DHK/