There’s a fair number of brands selling mg alone and fe alone. Maybe it’s hard outside the US? Or maybe places like target and CVS don’t carry them - those places do have limited options.
I sprained something in my lower back badly enough about a year ago that it took four trips to the chiropractor before I could get out of bed in the morning without lots of swearing. After that, it would seem to be getting better for a few days, then regress into spasms again. After a month or so of that, I remembered that Mg is supposed to be good for tendons and ligaments. I took a good dose of Mg that night, and it was significantly better the next day, and improved from there. That's the only time in my life that I've had a clear, major difference from taking a supplement.
https://www.cvs.com/shop/cvs-health-magnesium-glycinate-200m...