Please be tolerant with us. You are, after all asking a question of poultry owners who take their commitment to their fowl very seriously. And, "hand me down ducks" or not you did assume the responsibility of their care when you took them in.
Now, there are several things you can do. The last option, simply my opinion, would be the stew pot.
You are located in Western Washington. I don't know exactly where but I am reasonably sure that not too awfully far from you there is a rescue group that might be willing to take the birds off your hands. They would be the first and best option simply because the birds would then have the chance to live out their lives with some oversight from people that care. I know of a group in Austin, TX about 300 miles from me that takes them in, releases them in public parks and lakes and provides for their food and care ongoing, all with a permit and oversight from the city. That could be one reason that you have seen birds in the public areas that look healthy.
Option two, Craigs List, the rehomeing forums here on BYC, you local paper. Put the word out that you have some birds available, be honest, they are older and need to have the chance to live out their lives.
I have a total of 62 birds (chickens, ducks and geese) and we are a no-kill farm as far as those birds go. If a bird on our farm is intended to be a meat bird they are bought as a meat bird, raised as a meat bird and dispatched as a meat bird with respect, gratitude and compassion. They provide offspring which we can sell for income as well as eggs so we think that when the time comes they, like us, have the right to a nice retirement. Hopefully they will not outlive us because most people think we are certifiable.
If you were closer I would come get them myself.