I buy the KAL brand of Magnesium Glycinate from Swanson Vitamins.
Here’s the link (note that each tablet is 200mg; a “serving” is two tablets which is why they say 400mg):
https://www.swansonvitamins.com/kal-magnesium-glycinate-400-mg-180-tabs
I previously bought Magnesium Citrate from Swanson Vitamins. It was the NOW brand. The only thing I didn’t like about it was that the tablets were HUGE - bigger than anything else I’ve ever taken. The KAL Magnesium Glycinate tablets are also large but they are smaller and more rounded than the NOW tablets, so they seem to go down a lot easier.
Here are the NOW Magnesium Citrate tablets, if the large size isn’t an issue for you:
https://www.swansonvitamins.com/now-foods-magnesium-citrate-200-mg-250-tabs