If you need a GOST filter that will provide a tight and true seal then a Polish FP-5 is your best bet. However, while safe and asbestos free, as they’re still current production runs I believe, they are gonna end up setting you back by about 40USD without shipping. They’re also a pain in the ass to find.

If you’re looking for a filter that will screw into a GOST thread and you don’t need an absolutely perfect, but good enough seal, or even a cosmetic filter, I’d suggest an Israeli Shalon Type 80 filter. They’re NATO STANAG and are made to fit into the GOST threading of a gas mask. I’ve used my Shalon Type 80 on a friend’s GP-5 with the GOST threading and it threaded just fine and made a decent seal. Obviously I wouldn’t like set off a noxious gas or chemical and 100% trust the GOST- NATO STANAG seal, but if you’re using a GOST threaded mask then I’m assuming you won’t really be using it for a serious chemical situation or leak, more likely just a cosmetic thing. Type 80 filters are rugged, cheap, and still made even to this day.

Here’s the link to buy one off of Amazon, make sure your seller is reputable before buying or you might get an already opened and used filter:

NATO Israeli NBC 40mm Gas Mask Filter Only! (Original Version) https://www.amazon.com/dp/B01N1YRZG6/ref=cm_sw_r_cp_api_i_cCEGCbFZR6AR2