- Get Tor browser and find one of the various marketplaces, purchase small samples and send part of the sample to one of the various test labs that can confirm the quality
OR
- Find a community where people are likely to have some sort of experience with psychedelics, like self-improvement groups, hacker spaces or just visit raves
- Start talking with a person who could maybe possibly have done psychedelics at one point or another. Don't ask them outright about buying, but starts a conversation about your curiosity around psychedelics for self-help.
- If they reciprocate, you can at one point of the conversation ask them kindly if they happen to know anyone who can sell to you. If they say no, just move on with the conversation as to not make anything awkward for either of you
- Repeat until you find someone, you'll find someone for sure, in either of the groups mentioned before.
Clue: If they are a guy with long hair. Or if they are a gal they might have flowers in their hair
Change "long hair male" to "dreadlocks on a white male" and you'd have a point, but the majority of long-haired men I've known either wouldn't do drugs or are into non-psychedelics.
And having spent a lot of time in clubs/festivals/house parties around drug use, of the many many users and dealers of psychedelics I've met... I don't think I could narrow down a single "look" that's more, or less, likely than any other.
On the other hand, a) there is "a look" to facial expressions (not dissimilar to a weed "stoned" or amazed expression) that can be quite recognisable of somebody being under the influence - aided by things like super wide pupils and b) there are activities - such as being in a place where people are dancing to electronic music of some genre - where plenty of people use psychedelics and those faces can often be seen (though easily confused with expressions caused by other classes of drug).
Definitely always remember test kits are your friend.
Then just ask.
The drugs will come naturally after that (if you want them to)