I have been mentoring and advising early companies and startups for a while now... It is mostly done through some organizations - alumni or University mentoring services, state or local, or some specialized mentoring groups and services. Most mentors are volunteers and usually they are not allowed to be "compensated" by companies they mentor. Advisors on the other hand are usually compensated for their time (more often with equity and expense reimbursements).

If you do not know an appropriate organization that could help you find mentors and advisers - reach out to relevant experts in your field or academics. People usually are willing to talk and if you like what they are saying you can discuss formal arrangements.