1) My mom is a sonographer at a hospital. That way was easiest
2) I am involved with my schools premed club. I asked one of the docs we had as a guest speaker.
3) When I was volunteering in ER I asked one of the interns. havent shadowed this doc yet, but we've been emailing trying to figure out a day. Hes now a resident and in the nuerology department so i'll be shadowing him there.
Most doctors are pretty nice about it. Ask your primary care physician if you can. Talk to your schools premed clubs, they may have some docs that have previously worked with students, or have came and spoken at meetings. Good luck