This is where I got the sour salt first, in the kosher section of a large supermarket chain here. .
Before you buy at, say, King Arthur, check online for citric acid; it is sold at many places, and you can get it at a better price, but make sure you are getting the fine stuff as this is best for cooking uses. It will keep forever in a glass jar. I also keep some in the bathroom to take care of hard-water deposits in the toilet!