Meijer, a Mid-West regional Wal-Mart style grocery and department store, has started selling their 4 packs of 10% "Pool Sanitizer" for $12.99. I picked up a few cases today, which had a production date of April 10, 2017. The bottles are 128oz., as opposed to the typical 121oz. bottles that bleach comes in, so my less than stellar math skills tell me that is a better deal than buying the 8.25% bleach at $3 a bottle they sell in the cleaning supply department.
The cases of 10% go on sale for $10 a few times over the summer too, so I always keep an eye out for that to stock up.