I am looking for any information on how events search out and find speakers? Agencies or referrals?
Go through an agency. Keep in mind often you can book talent thought more than one booking agency. Have some criteria in mind. Budget, topics, time they will be needed, if you want to do an book signing if they are an author etc.
I have used:
Google should be able to help. Search for: Book keynote speaker, celebrity keynote speaker etc.