I (along with most people) buy bulk chemicals because they do the same thing and are much much cheeper
for calcium i use dow flake which is calcium chloride
for alk i use baking soda which has been baked for an hour at 400 degrees in my doser for slight adjustments i just use reg baking soda without baking
and for Mg i use mag flake which is a combination of magnesium chloride epsom salts
heres a good link to read
http://reefkeeping.com/issues/2006-02/rhf/index.php#4