I've just got myself some more Calcium Chloride this week. When you've decided
why you want it and
how much you might add, you would not go wrong getting what I got:
Calcium Chloride 33% solution (Ebay) (it's for cheese making!). 1L is rather a lot. You can get smaller quantities and most "water calculators" support using solutions now.
Calcium Chloride is terrifically hygroscopic, so buying the "solid" stuff is practically a waste of time: Having decided why you want to add it, how much you want to add, got your (dubiously) highly accurate measuring scales, weighed out exactly how much calcium chloride you need ... STOP! You won't have a clue how much water is actually in the Calcium Chloride! And therefore, no real idea how much Calcium Chloride you have very carefully weighed out. An aqueous solution is far easier.