Reading about these on another forum.... I have a set of surplus masks from the old days but the filters, while unopened, are long expired. I no longer live in an area that's in danger of chemical spills etc but it would be nice to have working masks for the totally unforeseen. When I've asked those as use the M16 models where I can get replacement filters, they always assume it's gonna be easy but then come up empty. Does anybody know where you can buy them?