I know that Michigan State has this, and there has been mention of the fact that there are programs there, but are there any more?
I’d suggest making a list of schools you’d be interested in attending, then going to their websites and looking up what sort of concentrations they offer. If you don’t come up with enough schools, then expand your list and repeat the process for the newcomers.
Keep in mind that not all schools have formal concentrations, so the real indicator you should look at is the research areas of the faculty. If you see a school doesn’t have a formal biomechanics concentration but there are a decent number of faculty who do research in the area, it’s a good bet that they offer plenty of electives in the field and would be a good fit for you. You can even check that by reading through the course catalog for that school.
One thing I observed is that schools that have a separate BioMed major from ME, don’t necessarily have a concentration in BioMed for the ME students. Seems counter to what you would think. That doesn’t mean you cannot take Biomed electives but it could be harder to fit in.