[roel_v]

Go the a big cities website, or the police department's, or a criminology researcher's. Find a map or report of areas where known drug dealers or prostitutes hang out (there is often overlap between these). Go there late at night in the weekend. Walk around a bit, don't make contact with anyone. After a while you'll get a feel for the environment and can make (short) eye contact with people. Dealers will pick you out, if they haven't approached you before.

I walk through parts of my city regularly where I am often approached in this way, even though I don't want to buy anything from them. So I only know about the first part, I don't know anything about any transactions after that would take place, or how to keep yourself safe. Maybe dressing like a bum will make the chance they'll mug you smaller.

Alternatively, associate with someone who most likely has some underground connections. Call and meet a prostitute, approach a group of youths hanging around on the street, approach people in a clubbing area, things like that. All of this I got from watching Law and Order but it seems reasonable enough :)

[king_jester]

If you don't like the super shady feeling that accompanies doing this, you could make friends with someone that uses marijuana. Contrary to popular portrayal, there are many users who are successful, normal people with interests that align with yours. Once you know people that smoke, you'll likely never be without at least one connection again (unless you move).