Animal protein is better! Birds grow thicker, faster etc etc. If you go to any of the heritage threads say HRIR thread, you can get a lot of good information on the difference between soy based feed vs animal protein based feed. Another thing, chickens are not herbivores, they are omnivores. Meat protein is something they actually need. A lot of people feed them chicken livers cut into small pieces and mixed with there feed. Again a lot of good info in other threads.
As for where you can buy it, good luck. I can't find any feed that has it. Purina Game Bird feed has it I think. Also I think there Breeder feed has it also. That's the only feed any where around here that does that I know of. I have 60 chicks and chickens so I switched to Southern States All-Grain feed. I get the meat bird feed for chicks and roos - 20% protein and the breeder/layer feed for the hens. The cheapest also. $14.95 for a 50lb bag. I can just throw some chicken livers in there every now and again. There cheap also.