Even better, see if your school's pre-health office or pre-med organizations have any connections. These contacts are almost certainly out there as you probably aren't the first pre-med student to pass through your university.
OP- I highly recommend this advice. Cold calling will waste a lot of time because doctors are extremely busy (as you know) and don't have time to return every phone call by a pre-med wanting to shadow them.This is preferable to a cold call. You can at least begin your conversation with "I'm a fellow alumnus interested in a career in medicine." The implicit camaraderie will go a long way in breaking the ice and make your first conversation much easier.