I don't think I've seen a SINGLE ONE in my playthrough so far, and I'm now in Act 2.
I've googled it and it seems that Feather + Knife is supposed to = Quill but this is not the case.
Spoiler:
By combining a knife and a feather.
If you can't find one:
Just grab somebody's pillow while they aren't watching and slice it open with your knife. Plenty of feathers in there.
It appears that a switchblade acts as a knife only sometimes; in this case I needed an actual dagger.