The most popular one I think is Chegg. But you can search for online tutors and look at the reviews and there's also math.stackexhange. And I agree with the #2 post. We are here to help.
Surelly we can recommend; Edaboard. If you want help to solve problems on which you are already working but have issues at a specific topic, the Mathematics section is the appropriate place, whereas if you are just seeking for someone to whom you are willing to pay to do the whole job, you have to post at the Job section.