The most effective use of a nerve agent is in a closed environment such as a building ventilation system or in a subway tunnel.
The vapor is NOT a contact toxin and requires inhalation, hence the utility of a mask.
There are industrial masks that have vapor filtration canister filters that are suitable for protection from chemical agents. They are inexpensive (relatively) and effective. They come fresh in sealed containers with a reasonable shelf life.
Military surplus is largely junk. I would not consider their use.
Hope this helps
Jeff