Professional Networking: How can I find on the internet an expert in a given field?
My answer to Professional Networking: How can I find on the internet an expert in a given field? on Quora
We launched a speaker directory for Lanyrd recently that might help here—it lists the most prolific speakers for thousands of topics, based on our dataset gathered across many conferences around the world.
We have a tech focus at the moment, so it will give you the best results for tech topics. This also assumes that speaking on a topic is a proxy for being an expert on the topic, which may or may not be true!
You can find other topics by searching on http://lanyrd.com/speakers/