Swimming-pool section of stores like WalMart and swimming-pool speciality stores.
Not all pool shock is the same. Many are made with other chemicals which will not perform the same way. They can also contain algeacides which can poison you if consumed in high enough concentrations.
This place sells it online at a 57% concentration. http://www.poolgeek.com/BioGuard-Burn-Out-3-P9506.aspx
As with other chemicals its important to store this stuff correctly. Its an oxidizer and will contaminate any food it comes into contact with or is stored with. It also will rust metals quickly. Store it in a mylar bag or a glass jar and keep it clean. Do not store it in the same cabinet or space as any food products.
As an oxidizer it also represents a fire hazard and it will create an oxygen rich environment in a fire.