You can feed them anything as long as you ensure they have enough protein, calcium and other nutrients.
Some use prepared layer feed.
Some people keep them on a broiler, grower, game bird, or flock raiser and just put oyster shell on the side. The big difference is increase calcium with varying levels of protein.
You definately don't want them on medicated feed.
Other mix their own feed (this has to be done with care because of the other nutrients needed).
You can do organic or not do organic, most feeds are not organic. There are many organic feeds available depending on which part of the country you are in.
I personally love CountrySide organics. We have a co-op and go in together on a pallet which greatly reduces cost and shipping.
If you buy one bag by yourself the feed is like 23.50 and shipping is almost another $20.
If you go in together and get a pallet call ahead for pricing you can get your feed for about $25 including shipping. Big difference there. If you are in an area that carries Kent or Blue Seal they are also very popular.
If you search these forum this discussion has many previous thread with lots of info.