KenK, pool chemical is a good option. This has been recommended to me for shock chlorination of my well. But in my case we're talking about thousands of litres at a known concentration. And that's the volume of a pail of pool chemical, at least as far as I know.

It's crazy that a consumer can't buy a simple jug of simple bleach. I hate to recommend Amazon for anything, but it comes to mind.

If long-term chlorination is not the object, sometimes hydrogen peroxide can be purchased in larger volumes from pharmacies or vet supply stores. Mixed with a little water and swirled around, it sterilizes bloody well everthing with great efficiency.