Modern versions of the mag that are produced for the Hi Power will work in the SA 35 - or to be specific, Mec-Gar ones will, and original Hi Power ones will (from the 1990's).
Older Hi Power magazines will in some cases be fitted with a 'mousetrap' spring on the heel of the mag. These were intended to provide a nice crisp eject of a magazine from Hi Powers that had the magazine safety installed. If you have legacy mags with the springs, you might have to remove them for compatibility with the SA 35.
On the matter of mags, I *believe* that Mec-Gar are the manufacturers of the SA 35 mags and Springfield just have them rolled with their name and logo, as did FN before they discontinued the original version of the Hi Power.
There were other manufacturers in the past, but if you buy a new mag, today, it'll be backwards compatible right back to at least a 1990's Hi Power, and probably back to the original Hi Power. Obviously, you'll usually find Mec-Gar mags cheaper than SA or FN.
Incidentally, as of today (8/7/2022) , Botach are selling 15 round nickel plated BHP mags for $20 ea.