I find mine at Lowes in the drain cleaning section. It is Roebic Crystal Drain Cleaner. I also have some on order with Paul, soapmakingman at his etsy site.
I get it at Home Hardware. Or if you have a country Depot where farmers shop for their farm stuff...many of those places have it. I think they use it to clean out outhouses.
Mom & Pop hardware stores have it. I get mine at a little True Value store a few miles from here. He even offered to order me a whole case at a time if I ever need it.