Welcome to BYC! Mountain Peeps has given you a good thread to check out. I'm a chicken person and don't know a lot about ducks (although I've had a few), but I have to admit that ducklings are the ultimate in cute. Good luck in finding some clothes for you little baby.
are you looking for diapers for your duck - is she a house pet. On the BYC buy,sell,trade forum there is also a category for "everything else for sale.," some people make diapers for chickens and I think maybe ducks. You could try looking there.