Comment by nerdsniper

I would generally use McMaster-Carr whenever possible. They will almost never go out of stock even during national crises. I don't generally trust Amazon for items that I can't validate myself due to counterfeiting, e.g. there's a lot of fake knockoff HEPA filters marketed as OEM.

3M 6000 full-face masks: https://www.mcmaster.com/5541T605/

The image shows just a particulate filter, but the listing actually includes 3M bayonet-style cartridges for chemical exposure.