Examples include people for keynote presentations and panel discussions. This is for cases where you don't have any direct or indirect connections to the people.
If you're looking for the best, that means you have a budget for hiring them. GREAT!
1) A site I recommend you visit is eSpeaker.com
2) You can also search for a company local to you (or not) that represent speakers. They'll provide you with the names and information of the best people for your situation.
3) Ask your audience for recommendations
4) Search using LinkedIn
5) Search using Clarity.fm!