Since you can't easily get feed, you might research your area and see if anyone has a home brewery going on and can give you the spent grains...very high in protein and fiber and you can feed small amounts of that mixed with other feeds. Fermenting any whole grains you use will give you more bang for your buck. Some folks set up black soldier fly stations in their runs...these always make me leery due to botulism. Meal worms are easy to grow, as are earthworms. Sprouting grains can make them more nutritious than feeding them dry, but fermenting them takes that even further so no need to do both. Growing crops specifically for them, such as pumpkins(I would hold these over for the winter and allow them to ferment), beets, kale, spinach, etc. can supplement their diet.
Doing a cultured deep litter in the coop and run can provide sources of natural forage for the chickens and also keep them healthier and more active while confined. Setting up places in the run where you provide habitat for earthworms or bugs with the use of old stumps or old bales of hay that can be flipped over now and again to provide forage for the birds is a possibility.
Foraging for spent fruits and veggies from neighbors and friends and even shopping marts in the area are also a good supplement to the diet. Feeding their shells back to them is good, digestible calcium, phosphorus and magnesium. Using sweet lime in your deep litter can also provide them with an extra source of calcium and giving ACV in their drinking water will help them to absorb the calcium from their dietary sources.
Lots of work to come up with additional feed besides the balanced ration of layer feeds....free ranging can take a lot of the work out of it, but unless your pasture/woods are exceptionally good forage you'll still need to supplement their diet a little, especially in the winter months.