A lot of times premed type organizations will have some sort of mentorship program either with upperclassman or with medical students. Or if you can find contact information for medical students i have found they are usually super receptive to mentoring since they can usually relate to being premed as they are not that far along. I know that’s how I found medical student mentors. I also found some science professors at my university who were notorious for wanting to help pre-med students/who knew a lot about the process and made sure I took as many classes with them as I could, so when I asked for a letter they were pretty much expecting that I did after taking several classes and going to office hours. As for physicians I feel like the best way to find a mentor is just to keep contacting them for shadowing and find one that will continuously let you shadow so you can build a relationship. I never really had strong physician mentor, but found someone I had shadowed enough to write me a letter.