Often the best way is -- as someone else mentioned here -- to approach bloggers who already write on your topic on their own blog. Plenty of experts have deep knowledge of their topic, but don't know how to turn those ideas into an engaging blog post.

We've got a database of about 500+ bloggers we mine whenever we need writers (we manage blogs for small businesses). Happy to put you in touch with someone who would be a good fit if you'd like: http://www.socialexis.com/need-a-freelance-blogger/