The problem with making tools, is that eventually you have all the tools you really need. I have met this problem and figured out how to conquer it! Simply manufacture the need! In this case I have decided to make a mandolin.
This requires a bunch of new tools! Since I want to learn luthery, I have started out by doing repair, adjustment and fine tuning. Repairing a range of instruments means even more tools.
One such tool is for cutting or adjusting the notch in the nut to take a string. Ideally you want a perfect fit. Of course you could just buy a set of nut files, but since you need a nut file per string size this could mean four files for a basic mandolin. If you broaden you range, you can quickly find a need for quite a few nut files.
Here is a small collection of string sets;