This selection of high quality knives is produced by Lee Valley. Notice that model E is the exact same knife, albeit reshaped, as is sold by John Neal Books as a quill knife. In its natural state it retails for about $18, reground $70.
Here are two others that are made by Pinewood Forge. With a moderate amount of grinding these make extraordinarily good knives for cutting quills.
This is the quill knife available from Kalligraphie.ch for 125 Euros plus international shipping, again looking suspiciously like a spear-point whittling knife available in the US for $25-30.
What sets a quill knife apart from a carving knife is not the quality of the steel, or the shape of the handle, but the way the blade is reshaped. Given this fact, if you're an enterprising sofer, who's handy with a whetstone there's absolutely no reason to go off and spend ridiculous amounts of money on a knife. Find a smallish, solid, high quality steel carving knife for a few dollars that feels good in your hand and reshape it according to the images and directions below.
This image illustrates very well the rounded back of the knife that is ideally suited to the sloping cuts needed to form the tip. The flat side on the other hand is used for the final vertical cut during nibbing.
Given the unusual shape of the knife, I thought it might be of some use to post a brief, non-technical description of how to sharpen it. There's no great mystery to it and you need only a few tools readily available at most hardware stores.
I don't think it really matters what kind, or even what size of whetstone you use and there are a large variety available you can choose from. For this demo I'm going to use my set of Japanese Water Stones. They work very well and put a razor sharp edge on even the hardest steel in minutes.
Pictured here are three different grits of stone. From right to left is the 800 which is my coarsest, followed by the 1200 and finally by my finest the 6000. These are available from Traditional Woodworkers Supply.
If you're starting with a very dull knife, or with a brand new knife from John Neal's you will want to start with the coarsest stone to begin building the edge.
Continue in this way for say thirty strokes, then lay the flat side of the knife flat against the stone sharp edge facing away from you. Now, with moderate pressure draw it toward your self the length of the stone. Let up on the pressure and let the blade glide up to the top of the stone and repeat the sharpening stroke back. It is extremely important that whatever you do to one side of the blade you should do to the other. So if you made thirty rocking strokes on the rounded side make thirty flat strokes on the other. I recommend you count them to keep the edge balanced.
When you notice a sharp edge beginning to form on your quill knife (however many strokes this takes) move to the next stone up, in this case the 1200 and repeat the process exactly. After you've worked it on the medium stone your knife will be razor sharp and you will most certainly be able to shave hairs off the back of your hand were you so inclined, but we're going to continue through two more steps to put a mirror finish on the blade. It isn't enough that the knife be sharp, it must also cut very, very smoothly.
Taking the 6000 grit stone it's necessary to work up the cutting slurry a bit with the small stone that comes with the set, like so.
Rub the smaller stone all over the surface and work up a nice thick mud and then begin sharpening the knife exactly as before. Incidentally, you can do this either vertically, horizontally, I don't find it matters. Thirty to forty strokes per side should be plenty, but if you feel it needs more, by all means sharpen to your heart's content.
Our last step is stropping. This involves drawing the blade gently over the surface of a leather strop onto which you have rubbed a generous amount of a stropping compound such a Flexcut. Strops and stropping compounds are available in many hardware stores as they are used for honing chisels, etc. Rub the honing compound onto the strop just as though it were a giant crayon.
To strop your blade hold your knife as pictured here at about a fifteen degree angle and gently push the edge away from you across the leather. When you come to the end of the strop, pick the blade up and lay it flat side down. Now with gently pressure draw the knife back toward you. Repeat. On no condition should you ever roll the edge of blade against the strop! This will dull your knife and you'll probably have to start over. As you strop you will notice very quickly black metallic streaks appearing on the leather. This is the stropping compound removing material from the knife and honing your blade to a mirror finish and that terrifyingly sharp edge we want for cutting quills.
It goes with out saying that once you're done your quill knife will shave hair with zero effort, it will also cut fingers so treat it with extreme care. When not in use I strongly recommend that you keep it in a small cardboard box all by itself. You don't want that beautiful edge you've worked so hard for to be damaged or nicked. John Wilkes, a professional pencutter offered this advice in 1799:
"Those who make their own pens have a great advantage provided they keep their knives in the best order possible, otherwise they can never make a good pen, for unless the penknife cuts exceeding smooth the pen will make a ragged stroke. The penknife should be kept free from all dust and damp...
...A good penknife, like a musical instrument, should be kept in the highest state of perfection, more particularly when it is remembered that a really good penknife is a scarce article."
Once you've reached this degree of sharpness you don't really ever need to take the knife back to the stones unless you drop it or damage it somehow. To maintain the edge, everyday before sitting down to work strop it fifteen or twenty times (takes less than a minute) and every cut your knife makes will be perfect. If you don't do this then your knife will dull over time and you will need to bring the edge back up with the water stones.
In closing, I'd like to say a word about cutting surfaces. I would strongly encourage you to use a slab of bone, horn, or a bamboo cutting surface to nib your quills. A glass or metal surface has no give and will dull your knife. Plexiglass might be ok. I use what's called a makta (a level slab of buffalo horn an inch wide by four inches long by a quarter inch thick) that I got from a Persian scribe.
Lastly, for whatever reason, sharpening is a very personal, highly opinionated subject and virtually anyone you talk to will have a different take on how to sharpen a knife. What's important is to find a technique that is going to give you a perfect, mirror-finish, razor sharp edge that will slice through quills like butter.