Where did you read about diarrhea from vitamins?

High dose vitamin C can cause diarrhea but that is usually 4 to 6 grams, 4000 to 6000 mgs or more, per day. 1000 mgs a day or even twice a day should not be a problem.

This is referred to as bowel tolerance. If the bowels get loose, one just reduces the dose.

The regimen should not cause diarrhea.

Magnesium oxide is a laxative for some. The other forms of magnesium are better.

I get mine from VitaCost.com. Just placed an order. Start an account and get on the emailing list and they will notify you of discounts. Start an order but leave it uncompleted over-night and they will email you a code for a 10% discount to finish ordering. The only thing I do not get from VitaCost is Vit E and NSAIDS. Walgreens has good values for those.