Hanchem here in Buford, Georgia, sells it by whatever quantity you like for US$1.55 per gallon. It's the 12.5% sodium hypochlorite. I just used fifteen gallons of it and it worked well. I never should have bought the fifteen-gallon drum, though, because I can't manhandle that thing.
Google Hanchem Products for the URL. My post count isn't high enough to include the URL.