For coaching, start by identifying what you want from a coach and ask around for recommendations from peers of people who match what you want. Do this first, before you start seeking a coach on boards like this one.

For mentoring, spend some time specifying what you're looking for a mentor, and identify people who are senior to you in similar, but not competitive careers. Then, conceptualize how you yourself might help the mentor in what he or she is doing, perhaps as a junior collaborator. Ask yourself the question, why would this person want to mentor me. After researching the prospective mentor thoroughly, look for a go-between in your network who'll arrange an introduction. Ideally, approach several people until you find the right person.