Like Trapper, I wouldn't touch calcium supplements. Reports are they do more harm than good, despite what the conventional party line says. Now that we know calcium supplements can impair Iodine metabolism, it makes even more sense to avoid them.
Magnesium enhances the absorption of calcium from food as well as Iodine absorption.
Magnesium citrate appears good but I'm sure there are other good ones like magnesium taurate.
Be sure to chech the FAQ for the other companion nutrients.