I like LinkedIn for a lot of reasons, and especially for the variety of groups available for members to join. My suggestion would be to choose the field in which you want to be mentored. Find a group that meets your requirements, and follow the posts in the group's feed. Finding a mentor is a very subjective, and it isn't something you should rush into. You can do a search on LinkedIn of the various groups to help you narrow your choices. Take a look at some of the members in the group and watch for interesting posts. Take a look at how various members reply to questions and the comments they add. When you begin engaging the various members of the group, you'll get a good feel for the type of person you'd like as a mentor. When you find someone "mentor-worthy", invite them to connect with you and start a conversation with them when they accept your invitation.
Perhaps someone else will have additional suggestions for you, but I've given you a good place to begin. Good luck in your search! There are, I believe, several million members on LinkedIn, and the right mentor for you is out there!