In my experience, there is no such thing on the market as "plain old calcium nitrate." Labels may say "Calcium Nitrate", but all calcium nitrate out there for gardening, again as far as my research on the issue is concerned, has ammonium as a constituent in the form "1% Ammoniacal Nitrogen". The brand I use, Greenway Biotech", says its label of "Ammonium Calcium Nitrate" is because California requires the use of ammoniacal nitrogen to be noted in its name. Greenway's (which is used in hydroponic gardening and is readily water-soluble) and all other brands I have checked, which is quite a few, include 1% ammoniacal nitrogen in the constituent label, regardless of whether or not the name on the item says it is "ammonium calcium nitrate" or just "calcium nitrate". That should make your search for what you need much easier.
Of course, if someone can give me a brand that does not include 1% ammoniacal nitrogen and is easily completely soluble in water (hygroscopic), I will gladly be receptive to that information.
Thanks