The best mentors aren't those who are super successful and noteable, they're the ones who have been through what you're about to do 3-5 years ago.

Using that filter, find companies that you admire the are growing and have a great brand around them - then approach the founders/CEO and ask them if they have 15 minutes for a call to talk about your business challenges.

If you don't have a business, maybe you can use a tough decision you're facing as the basis for reaching out - the key is to have a purpose and let them know why you think they can help.

If you present yourself right, then they'll usually ask that you keep in touch and let them know if they were helpful. Do that, follow up in 3 weeks - maybe in 2-3 months circle back and ask for another quick call or coffee as you'll be near by in a couple days (if you're not, change your schedule and meet face to face!)

Straight out asking "Will you be my mentor" is never a good idea. That relationships should just form naturally and it's on you to continue nurturing it over the years.

I've had many mentors over the past 10 years and I've never told them or asked them to by my mentor. I just turn to them for big / hard life decisions and always circle back and thank them for their time - when possible I mention them in press interviews - so they know how much it's appreciated.

They're reward is to see you implement their feedback and grow as an individual. If you do that, they'll always be there to take your call.