This forum has helped me out on many occasions so I thought I would share my success at sourcing an unusual cartridge load for shooting foxes and other vermin (not wildfowl!).
I have an old side by side shotgun that is not proofed for large loads (max proof of c.32 gram load and 70 mm length and certainly could not cope with steel). However as the gun is over 100 years old and on its second barrels I tend to stay below these levels. One day I will be able to afford a second heavy-load shotgun, but for now I have to make do with what I have been lucky enough to inherit.
I was looking for a heavy lead shot load (number 3 shot), 30 gram load, 67mm length, fibre wad with pretty standard velocities and pressures. Someone may correct me here, but I could not find this load, as standard, at any of the major cartridge manufacturers.
Caledonian Cartridge Company were very helpful and easy to deal with. They custom loaded cartridges for me at a fairly reasonable price of £60 for 250 cartridges plus postage. These cartridges are fairly similar to the "classic" cartridge on their website. I have used these cartridges now a few times and they are excellent.
I hope this helps someone?