The cable is described in the manuals, which are linked to below. It looks like it is not a standard ethernet cable (e.g., CAT6), but you should be able to buy some inexpensive parts and make it yourself.
Note that the preamp to power amp cable is NOT the same as the source component to preamp cable. Assuming that you are not daisy-chaining multiple power amps, it should be a 6-conductor cable with a 6-pin modular connector at each end, and a 180 degree twist in the middle of the cable.
http://www.marklevinson.com/downloads/downloader.asp?File=no331/no331.pdf&fileID=39
http://www.marklevinson.com/downloads/downloader.asp?File=no380/no380.pdf&fileID=11
Hope that helps,
-- Al
Note that the preamp to power amp cable is NOT the same as the source component to preamp cable. Assuming that you are not daisy-chaining multiple power amps, it should be a 6-conductor cable with a 6-pin modular connector at each end, and a 180 degree twist in the middle of the cable.
http://www.marklevinson.com/downloads/downloader.asp?File=no331/no331.pdf&fileID=39
http://www.marklevinson.com/downloads/downloader.asp?File=no380/no380.pdf&fileID=11
Hope that helps,
-- Al