You want a magnesium supplement that is highly absorbable. The one I recommend to my clients is
Natural Vitality Natural Magnesium Calm
This supplement is a powder that you mix with water and drink. It is a highly absorbable proprietary blend of magnesium carbonate and citric acid - which, in combination with water, creates ionic magnesium citrate. Two teaspoons contain 350 mg of magnesium.
The package recommends adding the powder to 2-3 oz of hot water. Let it fizz, then stir until dissolved. Fill your mug or glass with warm or cold water and enjoy. You can also simply add Natural Calm to a glass or bottle of cold water. Use only as directed.
Start with one half teaspoon (1g) daily and gradually increase to 2 slightly rounded teaspoons (4.5g) as needed.
Divide your daily amount of Natural Calm into two servings. Natural Calm can be taken with or without food. When bowels are comfortably loose, this is the optimal amount.