@Cuthbert Thanks so much! How do I begin? I volunteer at Jefferson in Philadelphia and I've emailed several medical oncologists/hematologists, but none have replied. It's been almost a week.
Also, note that a week might seem like forever in your life but in a big medical center with busy academics, weeks kinda fly by and emails come in droves and get triaged based on urgency.
As men2c mentioned, bigger places can have different policies in place and it might take awhile for someone you email to get in touch with someone who knows whether or not shadowing is allowed and how to go about setting it up given HIPAA and other regulations. So that might require multiple emails around to different people. It is vacation season now, too.
If your volunteering involves interactions with doctors start there. This is going to work better through people you have some face time with. Even if it's not the specialty you want to shadow. You can make connections and go along that way.
Quite a few academic or large medical centers often have ways to give people some exposure to healthcare professions, like through shadowing or other experiences. That would be something to look into as well, again even if it's not directed to just physicians.
One of the places I lived had a health professions shadowing program, not physicians, but a bunch of other fields. I wasn't a stranger to healthcare at that point but I did it anyway and learned a bunch and got to observe nurses in the ICU, rad techs, histo techs, dosimetrists, oncology RNs, etc. Because I was polite, respectful, and showed a genuine interest in their roles before ever saying anything about wanting to become a doc, when I did finally mention it to a few of these folks, they introduced me to MDs in their areas if they were available. I didn't even have to ask. I got the opportunity to ask the MDs questions, got business cards, even sat at a two headed scope with a pathologist for an hour looking at different biopsies and stuff. So I made connections for more in depth shadowing. The trick here is to show respect for everyone else's role and make sure not to come across like you think MD is the be all end all.
So call around and see if local hospitals have any health career exploration programs. otherwise do what others have suggested and start with people you know.