I buy a product called Fightin Scratch that my feed store gets from a mill in Guntersville, AL. It is loaded with Millet and BOSS and goes light on the corn. It is pricey....the 4-way is 15.20 a bag and the 7-way a bit more, but the birds love it.
I get the 4-way and mix some wheat and crimped oats in with it. The stuff TSC carries appears to be mostly cracked corn and none of my birds care for it at all.
I just picked up some IBC totes (275 gallon white plastic totes in metal cages) and am making some aquaponics systems. I plan on trying to raise fathead minnows in one of them (local fish farm sells them 60/1000). I will release several dozen into each pond every week for the ducks. Plus, I am growing duckweed, mealworms, and got a couple 55 gallon drums to make Black Soldier Fly larvae feeders (plans are here....
http://gardenpool.org/beneficial-insects/black-soldier-fly-composter-automatic-chicken-feeder) since the BSF occur here naturally.
Hopefully, that will help reduce the amount of commercial feed I have to offer the ducks.