I'm getting to shadow an ob/gyn because my mom visited him. It's better to have some line of connected, however thin, to link you to the doctor. Are there any other students on your campus who are currently shadowing? If so, ask them if the doctor would like to take you on as well. Often, when you email/call/visit an office, it is not the doctor you will be contacting, it will be a receptionist or office manager who has no clue what you want or they aren't super excited about having a premed kid running around their work space. What I mean is, your message may never even reach the actual doctor. Like I said, ask around. If that doesn't work, see if your advisement department has a list of doctors who have been receptive to students in the past.