Handloading. Make sure you get the proper propellant (the original is a black powder cartridge) for your gun.
Depending on the required amount and power, you could try to contact CAS shooters to get some ammo. It seems to be a fairly popular cartridge there. Be aware that CAS shooters do not tend to load ammo to high velocities.
If it isnīt broken, it doesnīt have enough features yet.