IBM 5161 - Cable
A description of the cable is in IBM's technical document for the 5161 at here.
Pages 1 and 15 (PDF pages 5 and 19).
Reported sources of non-IBM replacement cables are here and here, and part number 923-PCL-10162-1E at Mouser.
Whilst it may be tempting to purchase a very long cable, that can cause problems (more signal delay, more signal degredation).
Probably best to limit yourself to what IBM used: 1 m (3.28 ft)