For bass nuts, it's easy. You can cut a bass nut with inexpensive diamond files, which cost AUD$12 (US$ is slightly above half that amount at this time). Get a packet of small diamond files, which contain
- thin flat file with tapered edges to do the initial cut
- thin round file that tapers at the end to do the remaining filing. For thin strings use the end (thinner part) of the file and move towards the thicker part for the thicker strings
I haven't found any fret files here unless I order them internationally, so this is what I use (I've cut bone and graphite compound nuts with diamond files) and it works really well. Note: Some care must be taken when using the round tapered file to avoid over sizing the slot. But if the nut is approapriately marked, this won't be a problem.
If you don't know what they look like, have a look at this page which contains two excellent images that are large enough to see each file
http://www.sunva.cn/Diamondfiles-07.htm
Regards,
Michael