I recommend taking a look through both Pulse and the Groups on LinkedIn. Folks who have great expertise to share are often contributing solid posts. A well-structured and thoughtful post will also show you their communications tone and whether they approach the subject in a way that leaves valuable takeaway thoughts with the reader.

So, my suggestion:

Go to Groups and plug in a keyword for a topic you'd like to find an expert on

Scan for groups with good focus (Technology Entrepreneurs or Integrated Marketing, e.g.) and browse a few of the larger groups discussions. Cjeck the 'popular discussions tab

Engage in the discussion and ask if they would be interested in a speaking opportunity

Repeat for each topic you're seeking expertise.

Hope you find this useful - I have located many subject matter experts throughout the globe this way.