I get the Natural Calm magnesium in a health food store - maybe you can find a source on the internet. However, I woke up this morning thinking about you, and that the best thing to get regular is Dr. Schulze 's Intestinal Formula #1 (http://www.herbdoc.com). I took that for nearly 4 years, and when I stopped I was still regular on my own every day. You take one the first day and increase by one every day until you have a big BM, then drop back by one each day until you figure out your maintenance dose. His SuperFood is a wonderful thing too.
I found one site that said that you need magnesium if you have several types of problems, including low thyroid - and since I had my thyroid cut out because of thyroid cancer I do have that problem.