Go to a real locksmith(not Canadian Tire or the local hardware store) with your good key. I would ride my bike to the locksmith so I can do an instant test with the new one.
I have a K100RS key made from an Ilco blank. The numbers stamped into the blank are VO6 V77. I believe there are other blanks that will work as well. A well stocked locksmith should have the blank you need.
The factory BMW blanks are a hardened steel that are used for among other purposes bank safe deposit boxes. According to several locksmiths that I tried to get to cut one of those blanks, they require a special carbide cutter. I suspect the reason they use that steel is to discourage copying because, as I was told, those keys are for special security applications. One locksmith told me he only cuts those keys when they are ordered by banks and government agencies.