school, parties, friends-of-friends. i live in a suburb but it has an unusually large drug market going on because it's right next to Detroit.
edit: also, local events. for example, DEMF (Detroit Electronic Music Festival) a.k.a TechFest was just this past weekend and i know a ton of people who got majorly drugged up for it, haha. and the weekend before was the local fair/carnival with all sorts of rides and junk food and stuff, and there's always dealers there (including some of the carnies, hah).