Green vegetables all contain magnesium, the greener the better. The reason is that every chlorophyll molecule has an atom of magnesium at its centre. If you make your own bone broth, simmer the bones for over twenty-four hours, so that the bone becomes brittle enough to let the magnesium leach out of the marrow.
If you make sure you get enough table salt (10-15 g/day, inclusive of salt already in food), that will help keep your magnesium in balance (along with your potassium and calcium) as well, but if you find you need magnesium supplements, a brand I have heard recommended is Slo-Mag. But better to get your minerals from your food, if you can, because foods generally have them in a more bio-available form, whereas manufactured products often have to settle for what is easy to produce.