Not sure how to find mash that isn't GMO. You can make your own mash. I have a friend that feeds his hens crushed barley, kitchen scraps, and crushed egg shells. They lay just fine.
I need to get a crusher so I can crush wheat and barley. It is easy to get free grains at harvest time, just talk to local farmers and ask if you can fill some 5 gallon buckets from their over-spill near the augers or near where the combines unloaded into the trucks. Otherwise, you might be able to get a mash from a dairy farmer- they feed their cows a combination of grass and grain.
Of course, you could always grown your own grains. I've heard of some folks who planted trees that produce fruits that fall into the chicken run which cuts down on the feed bill. I'd do a search to see what bushes or trees would be most beneficial.
The founders would be ashamed at us for what we are putting up with.