Eric,
Scientific supply places supply very pure chemicals and charge an arm and a leg neither of which I have to spare. I buy calcium nitrate at a hydroponics store and it is inexpensive (I can't remember exact cost). It is hydroscopic absorbing moisture from the air turning into a slush fairly quickly. I store mine in a tightly closed container in the freezer. Magnesium sulphate is epson salts and available at drugs store, also inexpensive. Unless you are using all the fertilizer mixture right away, I would mix and apply each (calcium, magnesium, fertilizer) separately to avoid precipitation of some of the nutrients.
Chuck