Free range and pasture raised chickens eat a lot of forage. From mice to bugs and grass to your prize tomatoes, chickens will peck at anything. Add in your table scraps and your chickens are probably getting a pretty balanced diet. You won't need to worry too much about the proper balance of ingredients because the feed will be a supplement to their diet rather than a primary component of it.
Obviously if you are going to make your own homemade organic chicken feed you will want to use all organic ingredients. Beware of soy and fish meal, however. Soy has a high concentration of phyto-estrogen and this is of concern to some people. Fish meal can have a high level of mercury and that is something you want to watch as well. There are so many good ingredients that can be added to homemade chicken feed there is no reason to buy questionable things.
You should be able to find all of the following ingredients in an organic variety. Most, like lentils, quinoa and barley, are sold at grocery and health food stores and are available in bulk. You may need to run by your local feed store for a few ingredients, particularly the oyster shells. Any ingredients that are hard to find in your area can be ordered online.