You will probably be better off with a normal 12 tube scope as they are far more common nowadays. I cannot recall the last time I saw a 3/42 tubed scope. Mounts will also be easier in 12 to obtain.
A simple 4x32 with 12 tube should do nicely. This old Nikko Stirling 4x32 Sporting scope i acquired off "The Bay" for little money seen here sat upon my Brno Model No2. Although just doing a quick trawl through it there does not seem to many of this size on offer at the moment.