Unfortunately, snap caps are somewhat perishable if you use them for malfunction clearance drills. Plastic snap caps are virtually worthless for that purpose but would probably work for dry-firing. A-zooms are the the most durable caps I have tried.
I don't think I've ever seen them for sale individually. What good is one snap cap?
Just find a friend who reloads and have them load some cases without primer/powder. Fill the primer hole with some firm silicone based material or even a dead primer. Cover the brass with blue Sharpie and rock on.
i was just googling this and found a good site
Training Ammo, Dummy Rounds, Tactical Gear - ST Action Pro