Clarity might be a good point to start if you search for "social impact mentor" https://clarity.fm/search/social%20impact

Otherwise, there are a number of platforms out there, the best approach would be to research on Google based on your specific needs and geographical location.

Facebook also runs a social impact mentoring platform https://socialimpact.facebook.com/mentorship/ for Facebook groups focusing on social impact so it could be worthwhile also searching on Facebook for relevant groups.

Good luck!