Also, pretty much any large multi-day music festival with a campground, especially reggae. Suppliers are way less discreet at these kind of venues. You'd typically either get lucky and get whatever they say it is, or not and get something that has no effect; chances of something dangerous are low.
If you're in the Bay Area and ever want to attend a Phish show (or other good jam band), I'd be happy to introduce you to the scene. It's been a life changing experience for me and an integral part of my life for the last 15 years and am always happy to show people around.
And yet that's exactly what people do allegedly. Well, it helps to not ask openly around until you trust the other person somewhat (which even means getting to know them for a while).