You can find several versions of the duck costume online or at your local costume store. Or, with a little imagination, you can craft a homemade costume that's just ducky.
Start with a yellow sweat suit or pajamas with feet and add a white felt oval to the front. If the sweat suit has a hood, attach "eyes" cut from black and white felt. If not, add felt eyes to a yellow hat. For the feet: A pair of bright orange slippers or socks will do the trick. Finally, cut a piece of cardboard or poster board and cover it with orange felt. Attach it to the hood or hat to make a duck's bill.