how to find a physics tutor?


Sorry for asking random questions lately, but has anyone got experience of finding a tutor in a subject you're lacking in as part of your PhD? Right now I feel like I really need some basic physics / maths support (I'm a biologist by training). Thing is I am too broke to pay anyone and tutors are expensive. I don't think I can teach myself what I need to. I mainly need help with basic calculus and physical systems.
If anyone has needed a tutor for their PhDs, how did you go about getting one? Where did you look? I am at a complete loss as to where to even start...