I have two flocks of ducks. One small (4 ducks) that share a range area with my chickens with no problem. In fact, they are afraid of the chickens for some reason. My other is a larger breeding flock of 16 Cayugas that are in an area of their own to prevent cross breeding.
If you can not get duck or waterfowl feed you can use chicken feed but make sure it is NOT medicated. Try, if no duck feed is available, to get a mixed flock poultry feed. Most feed stores,
Tractor Supply, etc. have at least one brand available if not more.
As others have said, ducks are messy. They need water available any time there is food available. They really should have something they can swim around in (pond, large kiddie pool) they need shade in the heat of summer and shelter in the cold of winter and they need a pen/coop to be in at night to keep them safe from predators. They love to muck down into any little puddle that exists so your run/range will rapidly begin to resemble a lunar landscape. They poop out anything they eat within 20 minutes of eating it and their poop is not like a chickens...you can not just rake it up.
Most importantly they need the companionship of another/other ducks. A lone duck is a very lonely creature indeed. If you get more than one plan/manage your flock. One drake to at least 4 females is most comfortable. If you have not seen duck sex look at some YouTube videos first. Duck sex is not pretty and they are often referred to as the rapist of the bird world. Two drakes will fight one another no matter if there are ample females or not. It is something they do. So get more than one duck but plan what you get as best as you can.
And good luck to you. I love my ducks. They are silly and sweet. But, knowing what I know now about ducks I am not really sure I would not have gotten more geese instead.