Most local mills have gotten out of the business of making chicken feed because they don't have enough customers to make sense of carrying all the supplements before they go bad. Many of the supplemental vitamins, minerals and amino acids the mills use are only available in 50lb bags. It takes a long time to use up 50 lbs. of methionine or vitamin D3.
Making your own feed has been discussed here many times.
I've run the numbers many times and unless you can free range on some pristine forage, there isn't a good way to provide the complete nutrition in a bag of chicken feed at the correct ratios for less than the bag costs.
We (small holders) buy grains by the bag and supplements by the pound. Manufacturers buy grains by the trainload and supplements by the ton. It is the economy of scale.
With 80 birds, it may be possible for you to get feed by the pallet which should be cheaper.
If you had more, you could get one of the local mills to make feed for you specially and they'll deliver it in bulk. You'll just have to ask. You would need some kind of hopper that holds a ton of feed for bulk. A pallet of bagged feed would be better.