I found my 3 rescue ducks on Craigslist. They had been dumped on a pond and were being abused by the wild mallards so a kind person collected them and posted them.Where do you find rescues? I've tried, but can't find any out here. Most places out here (Northern California) are sanctuaries and don't seem to adopt, but rather only accept monetary donations.
My 4th was a word of mouth situation.
However I can always find rescue ducks on Petfinder in the bird section. Have you looked there?