Start scraping the paint off the cable. I have been purchased the 20 conductor cable for a R30E before. I believe the cable is called "Rockbestoes Perfect-A-Flex. I purchased it at a local electrical supply house called Kirby-Risk.
It may be 21 conductor cable but only use 19 wires to run the truck. I think there was some conflict over what the parts book told about how many conductors the cable has and what was actually on the truck.
My best advice is to scrape whatever black paint you can off the cable, plug that information into a search engine to find the manufacturers phone number, call them, and they can give you the phone numbers of their vendors local to you.