I think that I have only seen 2 or 3 Greener Police shells in gunshows over the past 10 years or more. I think I paid between $5 and $10 for the loaded one and around $2 or $3 for a fired shell. If you have the gun and want to shoot it, you can form the shells out of Winchester AA shells but not out of almost all other plastic shells. I believe the extra 2 firing pins can be removed fairly easily.
To form the shells I made a die up with a handle on it much like a shot dipper. Put the metal die in boiling water then use some sort of press to push the die over a 12 Win AA plastic shell. Count to 5 then push the shell out with a dowel going in through a hole in the base of the die. All of the field grade shells I have used, crumpled rather than conformed to the die
cheers mooncoon