As someone who once completely ruined a 9mm bore snake by getting it good and stuck in a .30 Luger barrel, I empathize.
Still, nothing wrong with a .30 Luger, as it is the original caliber for the gun. (The 9mm was a later evolution demanded by the German Army, IIRC.) The problem, as you found out, is finding quality .30 Luger ammo to shoot if you don't reload. Back before European gun laws homogenized somewhat, the peculiar Italian rule of "No Military Calibers" kept .30 Luger commercially viable enough that Fiocchi and Winchester kept it as a regular catalog item. Ruger even offered their P-89 in the chambering. Now that Italians can own pistols chambered in 9x19mm NATO, .30 Luger (and 9x21mm) are getting a lot harder to find.