Searching for maths tutors online finds people willing to teach up to A-level. I'm looking for help at a more advanced level.

At the moment I'm trying to teach myself category theory from downloaded lecture notes, but I have my eye on other mathematical fields including having another go at algebraic geometry once my category theory is better. However, because I'm teaching myself, if I get stuck I have nowhere to turn. By the same token, I'm doing the exercises but it's frustrating when there's no-one to tell me if I'm getting the answers right or approaching it at the right level of rigor; I find myself missing being able to submit work and get it marked.

How might one go about hiring someone who might be able to give occasional help, either online or in person (I'm in London) at this level? I'm sure university maths departments have plenty of people doing postgrad maths who might like occasional work like this, but how might I go about reaching them?