Start with physicians you know/are a patient of. If they can't/won't let you shadow them, ask for reference to another doctor they know who might. If that doesn't work, check with nearby med schools/hospitals and ask about shadowing physicians. Finally, if you have no other contacts that you can exploit, just search for nearby physicians and begin cold calling them to ask. Generally, there is nothing formal available to get shadowing opportunities, and no applications as such.