One starting point is to check with your friends' parents. I haven't done any shadowing yet, but I'm setting something up with a family friend who is on the pediatric liver transplant team where I live. I'm also considering asking a friend's dad, who is a hand surgeon, or someone else who is a pediatric neurologist.
If you don't have friends' parents who are physicians, or don't want to ask them, another place to start is with your own pediatrician. I'm kicking myself right now for forgetting to ask mine when I had my last check-up.
Also, I've gotten a job at a lab this summer, one that is attached to a hospital (literally attached, as in just a few hallways away), and the person I'm working for has offered to set up some shadowing opportunities with people she knows in the hospital.
Finally, one the hospitals near where I live has a job shadow program open to high school juniors and older. It's once a month and you have to sign up 4 weeks in advance to get a slot. The opportunities are for a variety of hospital personnel, including physicians, nurses, rehabilitation people and more. It took some searching to find it on their website, but I'm thinking about trying that out this school year, so you can look for something like that.
If all else fails, then yeah, get out the phone book or start googling. I just feel that to be really awkward, asking somebody you have no connection to if you can follow them around for a day.