The question is still open after 2 months, so decided to answer :)

First rule: do not try "to find relevant" mentors - you will put a lot of effort and the result is not guaranteed. From my own experience - the best advise I have received sometimes came from unknown mentors. Still remember the case when I was participating in StartupWeekend and organizers asked: "Which mentor you want ?". We had no clear preferences, but answered: "Anybody, except Mentor A" (he was local, he was not from the field we have targeted, etc). But later, by chance, we have met Mentor A, and he was sooooo valuable, he helped us to avoid some serious mistakes :)

That does not mean that you should not have any criteria. You should. Try to make the filter of 3-5 MAIN criteria. Try to search examples of his/her consultantions. Check the LinkedIn or just "google".

With "sustainable" mentorships - you are not going to marry mentor. Some mentors just help you solve one little problem and you move forward. But if you want something more sustainable then: a) never lie b) allways give feedback how you have used the advise and what was the outcome (I allways love that feeling when somebody remembers you after 2 or 3 years and attributes the impact to your mentoring session c) do the homework if you agree on something. Mentor will never be doing things instead of you. And even if paid he will be frustrated if you will be comming to the same question/situation day after day without any progress from your side.

Good luck !