These guys might have what you want (pre-slotted),
AllParts. And if so, Don (dklsplace, the LTG webmaster) is a dealer for them. He can probably make a recommendation.
Here's a link to
Stewart-MacDonald's page of pre-shaped nuts. You might find one with the right dimensions for the guitar you have in mind. If not, you'll have to do some sanding/trimming. Even if you do find one here, you'll probably need to cut your own string slots. Have you done this sort of work before? I think most guys leave that part to a luthier with the right files and experience at cutting the right string spacing and depth. It's about a $100 job, so you could save yourself some money, but if it's not done just right, well... I just know I couldn't do it. But my woodworking skills are poo-doo. I've thought about practicing on a beach beater, but not on one of my darlings.
Here's another source of nut material:
Luthier Supply
Good luck! Let us know how it works out.