The Speaker's Wiki : Guidelines for Conference/Event Organizers

A three pronged approach for finding speakers is suggested:

Broadcast a message to your community to find speakers, whether via email list, Twitter, your website, or other communication method you use to both promote the conference or event, and let people know the next conference cycle is coming.
Browse the Speaker's Wiki by topic, to find people that might be suitable for each topic in your conference or event. You can search on words you believe describe your event, at top of any page of the wiki. And you can go to the topics page: to surf topics that might apply. Speakers using the wiki tag themselves, but also, others tag them. So some combination of search and browse should help identify a number of speakers in topic areas, that you may not be familiar with but that would broaden and divesify your speaker's list.
Seek recommendations for both those you identify in this wiki that you aren't familiar with, and from those you know and who know your event, for "untypical" speakers to help diversify your event. Find out about those you aren't familiar with, in terms of knowledge, speaking style, and chemistry for your event, from people you trust, but go out a degree in your network, to go beyond the "usual suspects"