In order to find a good mentor you need to know what you need to be mentored on - that is, what do you want to get out of the relationship? As you are starting a startup you can possibly find a mentor in any type of startup-related activity (startup law, accounting, programming, marketing, growth-hacking, etc). Only by starting your startup will you figure out what explicitly you need help with and can then figure out who is the right mentor to approach.

So I agree with georgespencer - you don't need a mentor right now, you need to start doing things. As you do these things it gives you something to talk about with various people and then you can dive into a mentorship relationship.

Regarding bidirectional exchange - perhaps you are very good at some things others are not. You can use this to mentor your mentor.

As you grow and learn your mentoring needs may grow and change, so what you are really should be seeking is a community not a mentor. Go to the meetups / hackathons and make friends so you can join the community.