Magnesium-Mg2+ is the center molecule of chlorophyll ( like iron in hemoglobin )...So you can get it in most absorbable way from green plants in form of juices, smoothies and with least side effects.
I would recommend juices made on spinach, nettle, home grown fresh made wheatgrass or barley grass, parsley - green part,
green part - upper part of carrot mixed with root carrot.
Some practiced mix is preferable , maybe the ingredients and ratios in the mix could be referenced in Norman Walker's book but in time you could develop your own mix.
I mix in green juices also chia seeds or psyllium from time to time to get aditiona beneficial fats in first case and brush intestinals epithelium in both cases.
Good sources of absorbable magnesium is also quinoa seeds.
I can assure on that because after bad day when felt exhausting ( specially mental ) cup of ttightly cooked of sprouted quinoa seeds could improve mental optimism.
If you sprout quinoa and then just make little cook not on the boiling point...you can make extraordinary dish isterad of pasta and also instead of meat because it is rich in proteins and amino-acids.
Also for the magnesium source buckwheat is beneficial.
I would recommend powder made of fground buckwheat seeds ..you can make vegetable soup enrich with this powder or pancakes of buckwheat powder - because of moucus excretion after we add water to the buckwheat powder you need no eggs for these kind of pancakes...
Well you can even make more stronger dish with right proteins and magnesium sorces if you make buckwehat powder panckaakes or like tortillas but inside you put some cooked sprouted quinoa seeds with some sauce...of course compatible...this is very strong meal good for lunch ...and extraordinary source of magnesium.