Does anyone know where I can purchase the exact nut used in the Build Your Own Electric Guitar course? It states 3/16" TUSQ slotted but no info about length and height (and what about slot spacing?).

@nortski I don't think the exact specs matter that much. Shaping and slotting the nut is done very late in the process and you make it fit the guitar you just built, matching the neck width, height of the frets and depth of the nut slot.
I shape the nut myself, from a rectangular blank, and work out the string spacing with a special ruler. Like this one (there are other brands too) https://www.crimsonguitars.com/products/string-spacing-ruler?_pos=1&_sid=32cbaaaca&_ss=r
Generally I try to put the outer strings 3-3.5 mm from the edge of the nut and the ruler finds the other spacings.

You might have to reduce the height to get the action you want, but thats just a case of sanding the bottom face.