Some places that sell ducks include waterfowl specialists Metzer Farms, and the more general Stromberg's Chicks & Game Birds Unlimited. Prospective owners can buy directly from either company's websites.
Metzer Farms focus mainly on ducks and geese, though they also offer ringneck pheasants. The farm sells over 15 different breeds of duck, and their website includes a comparison chart to help prospective owners decide which breed is best for their needs.
Stromberg's sells chickens, ducks, turkeys, swans, and game birds from a number of different hatcheries. Ducks can be bought as adults, juveniles, or even eggs. However, Stromberg's will only ship domestically. They will not export live animals or eggs. They sell many different breeds and their website also offers an extensive selection of poultry and fowl equipment.