Go to a pool store in your area. I would suggest a BIOGUARD dealer. Bioguard tends to make the best quality pool chemicals. Buy their product called Burn Out 3 or Burn Out 35. It is about 98% Calcium Hypochloride. See link below.
I’ve found some online sources as well. However I prefer to filter the water and not just treat it chemically. The Berky water filters work great and last a long time.
According to one site Biogard had Lithium Hypochlorite and not calcium.