Good nut files are expensive, but worth their weight in gold if you have several guitars or get into setting up friend's guitars as well as your own. I've never found a really affordable set that's any good, and I always think it's something that should be made in China but doesn't appear to be.
I use Hosco nut files, and have added a couple of individual nut files to the 10-file set I initially bought to expand the range (UK example given): https://tonetechluthiersupplies.co.u...set-of-10.html
StewMac also sell their own nut file sets: https://www.stewmac.com/luthier-tool...lectric-guitar
They seem to have stopped selling them individually, which is a pity.
With the smaller number of file sets, it's best to go for the largest gauge you are likely to use, so if you use 9s and 10s, then always go for a set for 10s. As a slightly larger slot is fine, but too small a slot isn't. But a set for 10s often won't be suitable for 12s or 13s on an acoustic unless you buy supplementary files.
McCreed's set are about as affordable a good set as you are likely to get, though if you want a better match to some string sizes, you may need to find somewhere that sells them individually as well as in sets. Of course you can always mix files from different manufacturers if necessary to make your perfect set up.
But once you start using them, you'll wonder how you ever managed without. And then your setups become better, your guitars play like butter and you'd willingly play twice the price for a set!