Hello,
The non-profit I work with is independently producing a documentary series, which of course we'd eventually like to have aired on as many broadcast stations as possible when it is completed. I know that we can make individual submissions to the stations, like PBS, but I am also wondering more about how this industry works. Are there agents that take on material and then market/distribute it? Does anyone have any advice for best routes of distribution? Our project is funded, but we can not pay for air time to have it broadcast. We are still in production right now, so I thought it would be best to come up with a distribution plan as early in production as possible. I'd appreciate any thoughts, experience, or advice.
Thank you.

Try to contact distributors, channels, stations, etc before you spend one cent on anything. Give it a few months - talk to as many people in the industry as you can.