This is a good exercise for anyone to do at home. Take your local newspaper, cable news broadcaster, or ed tech publication and ask yourself, "who do they talk to?" My own experience is that we see a preponderance of corporate spokespeople, politicians, executives, other journalists, and 'experts' represented by agents and publicists. And be sure to look at the different sections - who are journalists talking to in the business section? In entertainment? Journalists don't go out looking for sources any more, they just scan the daily feed of press releases. But don't take my word for it; do what this public radio station did and conduct a survey for yourself - just be sure to ask about more than just race, gender or postal code - these are important, but they don't tell the whole story.