I have been teasing these for quite some time, and you have been asking! I finally have these quill watercolor brushes available for sale!
 
A few things to note:
The sizing on quill brushes works differently than on normal round brushes. For example a size1 quill brush is similar to a size 6 round brush, a size 3 quill brush will be similar to a size 12 round brush. The size 6 quill brush could eat my normal round brush as a snack.
Quill brushes hold a TON  of water. The bristles are also arranged in a way that makes them a little less predictable, but oh do they have such a wonderful point.
 
Details about this brush:
Size 3 Quill Brush
Synthetic bristles
Fun dark teal handle