Here is a short article on nitrogen rich food:
http://www.livestrong.com/article/313163-what-foods-are-high-in-nitrogen/
I think oats and lentils sound like great sources of nitrogen, but I am not sure how they would effect the taste of your brew. But it looks like some sort of seed is what you want.
Meats and other proteins are the best source, I think eggs were once used, until we learned about dangers of salmonella.
It might be best to stick to store bought nitrogen sources...