Many pool stores do stock 50 lb bags of calcium chloride (or can get it from their suppliers) but you have to ask them for it, particularly if they also sell to the pool service trade. Either the dihydrate or the anhydrous forms are suitable and it does not need to be food grade (but the dosing is slightly different between the two forms because of the weight differences). Tetra is a common brand that is sold because of its purity but not the only one. Dowflake (dihydrate) and Peladow (anhydrous pellets) now contain higher levels of bromide (because of manufacturing changes since the brand was sold to Oxidental Chemical) so it might temporarily create some bromine sanitizer in the pool but the only significant side effect is that, since bromine cannot be stabilized against loss from sunlight you might have a temporarily higher chlorine demand, similar to using a bromide based algae treatment. If you are testing your FC levels regularly this becomes a non issue other than the fact that you might have to add a bit more chlorine to maintain the proper level for a short time.