You don’t.

But if you have the drive, and are willing to take a small risk, you venture into the dark web and locate one of the handful of chemists/dealers who provide the world with its supply and buy in bulk.

Or you find a friend who has already done this work.

Go to one of these:

https://psychedelicsociety.org.uk/experience-retreats

https://www.mycomeditations.com/

https://synthesisretreat.com/

This isn't legal or controlled, but if you spend some time attending music festivals, you can come across just about and drug you would imagine (speaking about the US). LSD and mushrooms are popular at these shows. Speaking from experience, I would that if you have a personality that can handle finding drugs, then this would be less risky than buying from an online source.

I am curious about that too.

Go to New Mexico for ayahuasca or peyote based religious ceremonies?

step one is realizing that if you want to expand your mind you have to reject the systems of control that dictate to you which thoughts and behaviors are allowed.

put differently: the barriers to experimentation with psychedelics are the ones that you have constructed for yourself.

Look into 1-PLSD