Does your school have like interventional cardiology? They do a ton of diagnostic radiology---there was just a study that came out May 1 in Circulation about cardiac MR and its utility in distinguishing areas of infarction in NSTEMI.
I'm not sure where you are, but I'm in the midwest and I've gotten a couple of invites to radiology interest days at multiple universities. There was one in the fall in Chicago and one in the winter in Ann Arbor, plus my school had a Radiology career night...
I'd join the professional organization and get on the med student list-servs. A lot of these orgs have mentor arrangements.
Look up the Student Radiology Interest Group at the closest med schools to you, as well.
Also, when is their professional conference?? Go to that, network, meet people.
And definitely cold e-mail people. That's how I got on my current research study.
And---anyways---maybe your school doesn't have IR, but I'm sure there's a hospital near you that has one. You can never be too far away from an interventional radiologist. Just go shadow that person and then see where that takes you. At M3, you probably don't have time to start and complete anything ground brreaking, but I'm sure you could do a case report or a small retrospective cohort study............
Good luck! Bottom line: network.