I have a master's degree in mathematical finance at Boston University. There are doctoral students in this project, but the so-called doctoral students in mathematical finance belong to different teachers to guide their graduation thesis because of their different research directions. Many teachers are from the Mathematics Department of the College of Arts and Sciences, and some are from the School of Management. So you need to find the research branch you are interested in (mathematical finance is only a general direction, so there will only be masters), and then find tutors with similar research directions on the Internet and contact them.

In addition: In many schools offering master, you can find their schedules and teachers, and then start with the teacher's profile, find the tutors you are interested in, and then contact them.