Good luck with your search. 358 Norma Mags are almost as rare as a 358 STA rifles.
If you're looking for the extra velocity in 35cal, the most recent Speer load data for 35 Whelen is worth a look. 2000-MR was formulated specifically for minimizing pressure levels for improved velocities with heavy-for-caliber bullets and it does very well in medium bores. My 35 Whelen likes that powder with the 250gr load data, and it's a significant velocity increase for my rifle compared to what I was using previously. 35 Whelen reaching 2600-2700fps velocity with 250gr bullets is a noticeable improvement (and noticeable extra recoil too :-) ).
But, I'm a 35 cal guy and still want a 358 STA someday, so I understand wanting the unique caliber.