You ideally want to develop a trusted source, probably through someone you know
http://www.widespreadpanic.com/tour
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.
Discovered only a few years ago, it's way too new and untested. The effects are not the same as LSD, and reports of negative side effects are common.
Unfortunately they are commonly sold as LSD. Very sad and worrying.