Start with physicians you've gone to before and sent letters. I'm a big fan of stroking their egos a bit by mentioning a unique characteristic you saw in their care and how much you appreciate them as practitioners. Then break into what you want to do and ask if you can speak with them. If the interview goes well I'd come out and ask if you could shadow them for a bit, and most likely you'll get a yes.
If you haven't seen a physician in your area yet, call around asking to interview the physicians. Then take it from there