The pool places should have it. They just know of it as liquid shock. Ask them if they have liquid shock, and they will probably say they have it. Check the label, it should say somewhere between 10-12.5%. If you buy it early in the pool season, then make sure it is not leftover from last year (or it will be virtually worthless for house cleaning). Best to go with a place that also deals with hot tubs. They may have sold some of their stock all winter (I'm guessing that people use it in hot tubs too, though I have never had a hot tub).
