Either dichlor, or stabilizer. Undiluted dichlor is about 55% chlorine & 50% stabilizer (not a typo: I know the numbers total 105% -- very LONG explanation will be skipped). The problem is that stabilizer, as sold by pool stores, Walmart, Costco, etc diluted with various ingredients, some of which can cause real problems. Here are two undiluted sources:
24 lbs PoolBrand bagged shock @ Sams Club
Kem-Tek Dichlor 22 lbs @ Amazon
Stabilizer -- cyanuric acid -- is easier to find, but is hard to dissolve.

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