Most anything with a metric name, can be a paine to buy, the 9.3x74 is no exception. The case is not a very efficient one. It was mostly designed for the Drilling to keep the under/barrel slender. For a single shot rifle that is not a requirement.
A much better concept for your application would be a 358JDJ which is created from a 444 Marlin case. Thee 375JDJ is even better. Cases and bullets are easy to find and making the brass
is also very simple.
I have a 375Win in a Ruger#3 that I had thought to rechamber, but the 375Win is more than powerful enough for anything I hunt. This rifle has a 22" barrel and produces nearly 2400ft with a 225gr Hornady SP. That is almost as good the 9.3x74.
I would go for a JDJ in either caliber.