When you get tired of searching for the right pre-made replacement nut for your guitar of the correct material with correct spacing and height, you end up buying few of the blanks and everything is under your control.
The first one always takes longest, but after you learn how to do that, it is a standard solution for each other guitar. In case you cut the slot too low, there are hotfix solutions - shim the nut with piece of wood or use baking soda + superglue to raise the badly cut slot. Then buy a new blank and cut it properly - it does not cost a fortune and it is a major piece of the guitar.