What are the best ways to find mentors and how do we know that a person is good for us and what are the ways to convince them (like giving them percentage or money) ?
A good way to find mentors that are qualified are through incubators and start up funding organisations. Virgin Startup has probably one of the best mentor programs around. But it of course depends on where you are based. Meetups is another good way to meet people, especially if you do pitches you will notice who has an interest in what you do and can add value. Do not pay anyone for just advice if it not clearly gives a value that you can cash back on. Mentoring is a bit like a charity, people do it to help entrepreneurs, and if things go well there can be both financial and professional benefits for the mentor.
Your primary focus should be on what they have done in your field. Not what they say, but what they have done. Ideally, they will have taken a company from startup to monetization event. They should be able to provide $ numbers, and references.
Once you come to formalize the relationship, expect to give them vestable equity. If you expect them to work for free you just might end up getting 'value for money'.
It really depends on the field your looking for a mentor business, technology life in general. I will answer that question on the capacity of technology I found the best ways for finding a mentor is to attend conference where your field of technology is being discussed and network with the attendees or anyone who has given a talk that you liked and ask you question and try to connect some might be happy to help out others might not be interested that's one way. Another way might be try to find a mentor in the capacity of consultant who charges a fee there are plenty of those around
Look to your business plan, and see where your needs are. I would argue to find partners more than mentors. You want someone who can actively move your company forward. Yes that can be through knowledge and past experience, but if they are still relevant and current, i.e. have political power to add to their knowledge most successful entrepreneurs will still want to be in the game. They will be impressed and energized by your offer to involve them as more than mentors but active participants. Again...if you need them. Worst case scenario is you only gain them as a mentor more in the shadow. Eitherway you benefit from their experience and knowledge. But if they are worth approaching, you will likely benefit from having them involved in a more active role. Do not offer a mentor vestible equity unless they are representing themself in a board of advisor role or actively working to support financial advancement. Equity of any sort is valuable. Investors will see you as competent for offering equity to top tier board of advisors, but mentors? You might offer them a contract with you, as in a sub contract from your equity position with the company. But when you go into investor meetings and begin offering disclosure on those with legally contracted roles and stakes in your company, stating you have contracted out equity for a "mentor" looks a bit amateurish. Board of advisors...completely acceptable and respectable. Both show humility and openness to learn, but the more professional equity contract is for a board of advisors member.