Choosing the right supplements is a must. Cheap
brands must be avoided so avoid pharmacies
and Wal-Mart or Cosco
and go instead for good reputed online supplier like iherbs or try to find out what's the best at you own local health food and supplements stores.
Doctor's Best is by far the best magnesium supplier. Natural Factors are okay with vitamins and relatively good quality-price. Thorne Research, Food Now, Source Natural, Sisu are all good stuff.