True mentor relationships are organic things that grow over time. Be prepared to slowly foster and nurture the relationship for years to come.
1) Start building a list of smart, interesting, leading figures you come across in business, investing, & professional services. Eventually your list could run 100+ people deep & even go global.
2) Reach out with 1-2 very specific questions for start-up advice. Then work on staying close to the ones who are receptive and helpful. Ping them with periodic progress updates. And 1-2 follow up questions.
3) Some mentors have subject matter expertise, while others are good as a general sounding board. It's up to you to manage the relationship.