Hang out near the bus stop near a high school. You'll be sure to pick up some underage ones there. Plus you will be the pimp when you offer to drive them home instead of waiting for the bus!
One time when I was going to the liquor store a group of nasty 16(approx) year olds asked me if I could buy some vodka. I said sure, why not? They gave me thier money and I went in and bought it. It was good vodka.
I knew some girls in hs (wouldnt call them friends) who'd do handjobs, sometimes blowjobs, for complete strangers for booze. They would stake out this one particular package store and score every single time.