If the peacock has been domesticated, it will usually be alright with whatever commercial feed you give it if it fulfills what they need in a dietary manner of speaking. Free-ranging peacocks or wild peacocks will eat insects, seeds, berries, etc. However, for those who have a peacock, one recommended food to use is cabbage heads. Since not many like to eat cabbage all by itself, this shouldn’t be hard to come by. Reptiles and amphibians that roam your property might become lunch for the peacocks once you let them roam - especially if you’re going to use them to watch the property.
Overall, there are many things a peacock likes to eat. However, the safest bet to start is usually cabbage and some kind of gamey meat - elk, deer, etc.