FWIW, I recently started taking Nature Made magnesium gel caps - 400 mg magnesium oxide. They're huge but easy to swallow thanks to the coating. (I'm taking it to help with migraines, per something I read from the NIH.) Magnesium oxide is not as well absorbed as other types of magnesium, but most supplements seem to use it. I think the RDA of magnesium for women in our age group is 350 mg.

There are also food sources, of course, such as almonds.