Ed,
I use to light the flare on the platform I worked (now retired), the cartridges were a solid slug and fired from a 24” single barrel Greener. All cartridges had to be accounted for, with shots fired and remaining cartridges documented. When cartridge stock was low the offshore HSEA would email the HSE manager onshore who would arrange the purchase and logistics. I think they were purchased from Seals Cove in Aberdeen who also did the flare ignition training, it wasn’t a big number of cartridges, couple of boxes a year. You could try making contact with the HSE departments of the major players, offering your services or Seals Cove.
ATB 243 Stalker