My Grandma used to use chicken feathers and made her own. She wasn't Eastern European (seems to be the primary users/sources for feather pastry brushes these days) - she just used what she had at hand.
I can't find "duck" feather brushes - but here are a couple of sources for "goose" feather brushes I found by googling on "feather pastry brush" ... should be the same. You'll have to scroll down the page to find them.
(I've bought from them before and have nothing but high praise for them)
2) Otto's Import Store and Deli
(this site can be hard to navigate)