El Cronometro is a good exam prep book if you're learning on your own
I haven't yet taken the C1 exam but do intend to sit for it some day. As regards teachers, you could click on the Find a teacher tab on the top of the page and search through the options available. One teacher that I know of is Alex ( https://www.italki.com/teacher/1758597 ). I liked the videos she uploaded to Youtube (like this one: https://www.youtube.com/watch?v=cguRfeWTKHo ) and that's how I came to know that she is on italki as well.
She has been a DELE examiner, so she'll be able to give you great insight into what they look for in a candidate. However, I don't have any personal experience with her or with any other teacher.
Good luck for your preparations!