It's not as easy as just getting a longer one, because the SCSI ribbon is part of the custom slide-in rail system of the CC/5xx series.

When I needed another SCSI connector on my CC-Zip I stripped the stock SCSI cable out of the CC's wiring harness and pressed a longer one in its place.

If you're good at soldering, you could probably make a little two-socket board... Or you might be able to talk one of the solder-wizards around here (of which I am not one) into making it for you