Ｓｏｍｅ ｔｉｐｓ ｔｏ ｆｏｌｌｏｗ．．．
-Old Russian masks aren't the best, especially the cheap GP-5 masks; it's hard to see if the canisters are working and they may contain asbestos (though it won't cause harm if the canister is in good condition.) On a side note, Russian masks like the GP-5s are hard to put over your head because it's mostly rubber and will pull on your hair.
-Western masks and Russian masks both have 40mm threads, but they're not interchangeable; there will be gaps and holes or they just won't fit. If there are gaps, it will make the entire point of a mask moot.
-Make sure to take the tab on the breathing part out or else you'll choke.
Get one with a good fit; make sure to measure your head (circumference). A good fit means a good seal, and a good seal means higher chance of not getting gassed.
-Civilian gas/toxic fume masks are pretty cheap and sturdy; they're tried and tested by hundreds of thousands of workers each day in factories, plants, and so on. Make sure to get the right filter for the occasion if you do; it's pointless to have an asbestos mask when protecting against CBRN.