Is there some reason you don't want to use commercially made feed?
If you really do want to roll your own then I'd start with my local feed stores. What grains in particular they may stock varies from store to store and region to region so there's no way to know until you ask them.
If you're in an area that has no feed or farm supply stores then you can get by for a time with your local grocery. I use rolled oats (quick type for very young chicks, thick-rolled for older ones) as chick scratch in my brooders. They like it well.
Do you have a recipe already for your home-made feed? Most especially for chicks it's important that it be nutritionally balanced.