Epsom salts can be found at most any pharmacy or drugstore. They're used by us humans as muscle soaks or as a laxative (nasty). They shouldn't be very much, just be sure to get ones that don't have any scents or colors. Plain magnesium sulfate is all you want.
ES is different in that it isn't a true "salt" like AQ salt is. I think we just call it that because of its crystal structure or something.