Walmart in the Pool Section.
Its called Stabilizer and Conditioner. Walmart carries HTH which a lot of us use, but any brand is fine.
You dont need both liquid and dry. Pool math says liquid OR dry... Just get the dry. Liquid is expensive and not as potent as the dry.
get 2x .. over the course of the summer, teh CYA will probably drop a bit from backwashing, rain overflow, splashout etc.
It doesnt go bad either, so if you dont use it all this year, you can carry it over till next.
Divin Dave, super thanks!