Its a very simple tool that can be made easily, although some woodies for whatever reason prefer to buy one. I guess they are not set up to make thier own.
The pic shows it with no handle and little piece of tube is the ferrule. The turned down portion is for the tang, whereas the smaller project is a piece of HSS tool steel 1/4" sq that acts as the cutting edge
Heres a good discussion from another forum on this type of tool