You can use unpasteurized apple cider vinegar with the mother as a starter. Braggs is a common brand found in many supermarkets. The Walmart here has also been carrying it. Once it's fermenting you don't need to keep adding vinegar. There is a whole (huge) thread on fermented feed:
I love it when you guys post things that are easy to get to.
My DH Figured out who is laying the light brown egg. It is Faith our EE. I was hoping she would give me green eggs. Somebody said that if their ears are brown, they lay brown? Huh? So our silky, Karen has blue ears. Blue egg??
https://www.backyardchickens.com/t/644300/fermenting-feed-for-meat-birds