The best way to find a mentor is to look around in your industry and see who you feel has accomplished what you're setting out to do, and that you gel with the way they did it (values, brand, etc).

"Ask for money, get advice. Ask for advice, get money twice"
~ Pitbull

If you can get a warm introduction, do that. If not, you can always cold email them with a specific issue that you want advice on .. ideally it's something super relevant to their life experience and interesting enough to get their attention.

Never ask them to be a mentor. Mentorship usually develops over a couple meetings and is informal.

The value to them is to give back to the next generation. As long as you listen, execute on their advice / or not, AND follow up - then they'll continue to give you some time.

Mentors are best for those large life decisions that most people turn to their parents too ... those decisions are interesting for most noteable people to give you 15 minutes on a call to discuss.