You can't.
The 9.3x57r360 (full name of what you have) is also called the British .360 2-1/4 express, also known as the .360 miniature express.
This website will make you dies:
http://www.ch4d.com/This site will sell you brass:
http://www.buffaloarms.com/Bullets can be purchased but you need to do a chamber cast and pass several slugs through the gun and get your measurements correct. Your gun's bore could be anything from .36" to .375+.
Incidentally, this will be a major financial and time based undertaking for you if it is a drilling or double rifle. If your "combination gun" is a Cape Gun, you really don't care about regulation and therefore it may be easier just to shove different loads in and see how they pattern.
