Magnesium Chloride is the cheapest, magnesium oxide has the strongest laxative effect. Take between 5-10 grams per day spread out throughout the day. That will yield around 625-1250 mg elemental magnesium, 1.5 to 3x the RDA, but you likely need to it up from a deficiency. There is no way to overdose (gut rejects it automatically).
Thank you for your insights. I will try that next vacation.
I suggest you talk it over with your doctor first.
I have been told that oxide is not very bioavailable at all, Magnesium Citrate or Glycinate are better.