First of all, you're lucky! The SM194 is a FANTASTIC monitor.
For a cable, just try to ask a dealer like Emmesoft (www.emmesoft.com), Mucs (www.mucs.com) or Seidel (www.seidel-soft.de). they should sell that kind of cable. Please note that I am simply an Atari user and I'm not concerned in any way with the dealers above.