There's calmag plus by Botanicare which is pretty popular. There is also CaMg + by General Organics (General Hydroponics). I have both but only use CaMg+ since I am mostly organic. Others make it as well.
Molasses is a great source of cal mag too as well as other trace minerals.
i was about to go buy cal mag today but decided black strap molasses was the way to go after checking ingredients. i found one bottle of waterd down cal mag with a 4:1 ratio calcium magnesium but found the same ration in the molasses along with beneficial carbs vitamins irons and a few other things.
well see if it helps my cal mag def problem
also being on a strict budget a waterd down bottle of processed molasses for 20 bucks or a concentrate for 6...it was a hard choice especially in soil!