filters that come with the surplus gas masks are expired (either ask the seller or see if the exp date is put on the filter).

You usually by these on eBay, but i'm sure you'll find local offline and online stores that sell these too! and remember it doesn't matter if the surplus gas masks are decades old (i recommend not buying gas masks that are more than 4 decades old and don't buy gas masks made by the USSR because the filters that they use contain asbestos and they have a different connector so you can't buy new filters for those masks)

these things are built to last! so don't worry if you can't buy recently issued ones. You only need to worry about the filters! (you usually don't need to worry about the filters, but since you're going to use it to filter tear gas then you do)

Godspeed.