Ask any doctor that treats you. That's always a good place to start. Also, use any and all connections you have? Mom/Dad a nurse? Best friend's mom is a PA? Ask them to hook you up with a doctor. Premeds often do the wrong thing when looking for shadowing: she/he thinks "I'm going to be a pediatric interventional cardiologist so I need to shadow one" and the limited amount of them either don't respond to e-mails or politely say no. While its very admirable the student is so goal driven, they sometimes can't find the ideal opportunity. This is where you need to compromise: Take any primary care doctor. Peds, family medicine, IM are all great doctors to shadow. Maybe an IM nephrologist will let you shadow her/him; its not peds interventional cards, but, its something.
E-mails are ok, but be prepared for lots of silent rejections. If there's any local private practices near your home/town, those are much easier to shadow in. I personally just showed up and asked the receptionist if she could give my letter to one of the docs there to see if anyone would let me follow them around there for 1 or 2 days a week, and one wanted to meet me, he said I was welcome to shadow him, and that was the end of that 😀.
Also, if you had med student friends, they all know docs and can hook you up.