Unfortunately, I think they only come as an assembly with the capacitor. I was able to find phenolic washers that are almost identical and the nut and washer is something you should be able to find locally. I can send you one of the phenolic washers if it will help.
You could call Aircraft Magneto Service in Stevensville, Montana.
They go through about 12 or 13 hundred Slicks a year, so I'm sure they have a few spare parts.