Owen,
I also had difficulty finding cables for my C8. I am sure you could fabricate something from commercially available brake cable parts that would do the job nicely, but if you want something that looks original, than it's a bit of a challenge.
Both the C8 and C15 (and also C8A) seem to have used cables with a eyelet at the end, while most Chev cables at the time had a threaded end (again, you could probably make that work). NOS Chev brake cables are offered on Ebay quite regularly and even though I haven't quite narrowed it down, the eyelet type cables seem to have been used from 1936-1940.
Have you checked if a shorter cable would work? Cables with eyelet end and about 31-32" overall length are offered on ebay regularly....longer ones are a bit more scarce. I bought two of the cables attached for my C8 recently. They are 38-1/2" overall length according to the Master parts list....which will hopefully work on my C8. There is a Left and Right version, but I have only found two Left ones so far.....so, if you find two right ones, let me know and we can do a trade
Hope this helps!
Alex