If you are referring to old gas masks, watch for auctions on eBay. Collectors of military masks would likely exhibit at gun shows, so check there, too.
Check online auction sites, like eBay, as well as military surplus stores. It isn't clear if you're looking for a specific period, but larger surplus stores can direct you.
You can go to any thrift store in any old military town, those stores usually have all kinds of old and new gear. You can also try an army surplus store or search ebay.
