Students who've taken systems programming classes (e.g. Operating Systems) would be your best bet -- in my experience, most low-level programming is shafted to those classes, except in the cases where a professor arbitrarily requires C, which doesn't necessarily happen in algorithms classes either.
But even better would be to find students who hack on open-source projects written in C (e.g. Linux-related efforts) -- that's much more representative of a graduating student's ability to be productive using C. There are a lot of students working on such projects, and it might be worth starting there.
But even better would be to find students who hack on open-source projects written in C (e.g. Linux-related efforts) -- that's much more representative of a graduating student's ability to be productive using C. There are a lot of students working on such projects, and it might be worth starting there.