Nut files are expensive, but essential. The files you get from Warmoth, StewMac, and a few others are the good ones made by Hiroshima Yasuri. They're about the best you can get, and worth the money. Fortunately, there's a company up in Canada called
JAPARTS that sells those files without the private labelling at a much more reasonable price. You can buy individual sizes, or they have sets to match string gauge sets.
They're not the one-stop shopping outlet for luthiers that StewMac, Luthier's Mercantile or Philadelphia luthier are, but they do have a bunch of other files and stuff for sale, too. You might want to check them out.
A side note on the nut files - the finer gauges are surprisingly easy to break as the metal is somewhat brittle and necessarily thin. One of these days, somebody's gonna come up with a file holder that'll keep those things from flexing and they'll probably sell 100K of them the first year. In the meantime, either buy spares or learn to hold them in the middle so they're less likely to flex. Go slow on the draw, don't push them, and don't try to cut too much in one round. When you get close to the depth you want/need to be, only do one or two pulls and check your depth. You can always cut more, but you can't put it back if you cut too much. It's quite frustrating to spend an hour fabbing a nut only to overcut the B string and have to start over. It's called "blowing a nut" and you don't want to have to say you did that out loud :laughing7: