When I saw that Great Plains sold Williams ribbons cables I thought about just replacing a couple of mine on High Speed and Cyclone too instead of buying a crimper and the parts to repin the old ones (and because I thought that sounded a lot harder than it turned out to be).

However from what I could see, Great Plains was just selling WPC cables, not System 11 - although I could be wrong - it couldn't hurt to ask them.

Also, from the Williams Blue parts catalog at Planetary Pinball:
http://www.planetarypinball.com/reference/partsmanuals/WMS_Parts_Blue_16-9064-B/files/mobile/index.html#43
...it looks to me like Williams only sold the "backbox cables" (H-10972) as a set - I couldn't find any individual part numbers for the individual Sys. 11 cables.

I ended up buying a crimper and some various pins and crimper contacts, all from Great Plains, and that has worked out great for me. The guy at Great Plains was super helpful and knowledgable too so I bet if you tell him what you're trying to do he can hook you up with whatever you will need.