When I had some keys cut I went to a locksmith who cut keys for bank safe deposit boxes. Those keys are steel like the BMW keys.
I would think that if you ask at your local bank, they might be able to tell you who can cut your keys.
Regarding the "cartridge", they locksmith who cut mine said there is a special tool involved. He didn't have it, but was able to make the keys with the tooling he had. Can't say if it was the cutter or the fixture for holding the key. I think it is something to do with the steel blank. While our keys are steel like the bank keys, there is a difference in the shape.
He also told me that ILCO has a blank that will work for BMW keys. It's not a common blank, but if the locksmith has them, they can be cut on the standard machine.