Keep in mind that there is no such thing as just calcium, aside from calcium metal that would catch fire if you added it to the tank.
All salts, like calcium ion, come with some counter ion , like chloride or sulfate. Chloride is to be preferred if you only have one.
So in general, calcium chloride is a very good choice, but a mixture of chloride and sulfate is even better (but not 2:1).
This article has more:
Do-It-Yourself Magnesium Supplements for the Reef Aquarium
http://reefkeeping.com/issues/2006-07/rhf/index.php