For one thing, people who become masters want to teach. If there's anything more fun than using a skill you've mastered, it's teaching that skill. But most people are not open to teaching. Sometimes they just end up talking to themselves on Twitter, reaching an audience that can't quite understand them.
Proteges also offer another perspective on things. It's useful for mentors to bounce ideas off them, or even someone who can see things from a more naive, open minded perspective. Someone who has all these ideas but isn't yet cynical.
So mentors are actually looking for proteges, even though they don't advertise it.
I think all you need to do to find a mentor is to do things in a specific direction. Eventually you bump into someone who is impressed by your ambition and chooses to be a mentor.
Most instances I think people find mentors while at work.
I have heard of a few services out there where you are able to book "x" amount of time with someone to talk about work or whatever (basically a mentor).
Since finishing up school I've worked with 2 developers, and both have been great and I've learned a lot.
I don't think these sort of relationships count as "mentors", or at least it just felt like co-working. So, I guess you could say I'm still looking? :)