I get mine from the local poultry farm.
There's one real fly in the ointment with quills, and that's the knife. You
can get by without a proper one, but it makes life difficult and precludes getting a really nicely shaped cut. Last time I checked, the cheapest purpose made quill knife on the UK market was something like £50, which is fair enough if you can see yourself using it regularly for many years, but a bit much if it's a matter of dipping your toe in the water. My suggestion is that you keep your eyes open for an old pocketknife with a small, well worn blade which is not hollow ground.
For information on curing and cutting, there's an article by Donald Jackson in
this book which is pretty much definitive. For a good demonstration of cutting, here's
a quick video by Ewan Clayton.