Domestic farm ducks, geese, some chickens and turkeys are white. This time of year is when they might be available.Check out poultry farms in your area. Wild game with white feathers include snow geese and swans. You might be able to find some molted gull feathers depending on where you live. Check local laws for potential infractions with possession of any of these wild bird feathers.
You can also post in the trade forum and see if anyone may have some they'd part with.
The stronger the bleach or peroxide solution the more it destroys animal cells. You're better off with a mild solution and a longer soak. Often times exposure to bright sunlight after chemical whitening can help further it along even more. This holds true for bone, horn, antler and most plant fibers anyway.