Anyone know where I can get a 12-string nut for nylon strings? Or any websites were I could order a custom cut nut? The local guitar shop near me does real shoddy work and I don't want to take this to them.
Are you going to string a steel string 12 string with nylon? Because for various reasons, it doesn't sound like a terribly good idea. (IMHO, of course Your results may vary).

There is a reason you can't find a nylon string top nut for a 12 string. Primarily because I can't think of a nylon strung 12 with commercial availability.

If you insist on moving forward with the project, you may very well have to hit up StewMac for a $100.00+ set of nut files, and DIY.

I think Godin has a nylon strung Oud. I think these have double course strings. You may want to get the neck width of that instrument as a guide. (If I'm incorrect, I apologize. I'm posting this from a not so good memory).