To participate in the BYC Buy,sell, trade forum you have to have 20 posts. Then you would go the forum and check out the thread called "Everything else for sale." There are usually two or three folks offering diapers or chicken saddles. Their ad would list the price whatever particulars they would need for instance, size of duck, breed etc. and how to order.
Ducks make even worse messes than chickens. You would be needing to change them and clean them up very often. If the duck spends most of it's time outside, you probably could get by with a diaper or two if it comes into the house just to sleep.