There is one tested and verified source in EU, Jake has not been around lately, we bought from the same source, and he actually tested it with his own methods, explained in SN megathread. It really is chemically pure, no anti-caking agents or other fillers. Even if rubbed on wet hands, it makes me dizzy, through the skin.
From experience, do not buy anything that comes in a plastic bag and is rated being "food grade". Better safe than sorry. SN's lethality and peacefulness comes from the method, the preparations concerning anti-emetics, fasting, and the acid reducer actually is important, when it comes to absorption rate, the speed of action.