I just use the same feed as I use for my poultry, but I do add more mixed corn or wheat to it. There are some propriety waterfowl brands out there but I have never bothered.
I use whole wheat and Fancy Feed Waterfowl pellets, plus mixed corn as an afternoon treat at times (esp when cold). The ducks prefer the whole wheat, but do enjoy the pellets if thrown in a shallow tray with water and some mud.