Feathers can be gotten from bird corpses. Birds of various types can be found in most cities and terrains.
Feathers are used in Finneblaugh's Thaumic Float and Gryntard's Feathery Reliever.
Owl feathers are used to make intelligence tea.
Duck feathers are used in the spell Granny Lipintense's Layer of Lard.
Chicken feathers are used in the spell Memories of a Vicious Chicken.