Okay, so asking to shadow can be a little scary but it is best to just do it. In my experience you need some kind of connection. Either the physician is a family doctor, a family friend, works for a charity you work with, someone who you have interacted with, etc.
Then call. When you get their office manager/secretary say who you are and what you want, then leave your number. If they respond cool. If not, then call someone else.
The best thing about most physicians is that they want to teach and educate. Most physicians shadowed someone at one point in time and most feel they should repay the favor.