To find shadowing positions I literally just emailed physicians at the hospital closest to my undergrad institution. If I did not receive a response after 3 days I emailed a different physician. I shadowed a Pediatric ER Physician (16hrs) , Neurologist (16hrs) , Cardio-thoracic Surgeon (16hrs), and an Endovascular Neurosurgeon (220hrs) this way. Published a paper with the Neurosurgeon I shadowed.
If the physician you are shadowing publishes frequently see if you can help out with some aspect of the project (most likely data collection, or chart analysis) - you could end up with a publication out of it. This shows great initiative.