Hi-

This might not be the most popular answer, but I love going with social data in a situation like this. Tools like BuzzSumo, FollowerWonk and Affinio are all ripe with data that can tell you who is active in the space associated with different keywords, topics, etc.

That is a general answer. To give you something more specific I ask, what is the exact data you are looking for? Do you know how you are going to apply it?

The social data can tell you who these people are, what other topics they like, what content is popular, etc. I love that as a starting point, but would love to know more about your goals and objectives.