You may want to consider picking up the docking station option.There is a repeat seller on EBAY who sells new,unopened docking stations. Included with your needed serial cable is a new power supply and the docking cradle.
All can be has for around $40.00US.
It's probably your best bet.
Another alternative could be to again look at EBAY for the printer cable accessory. These can be found for less than $10.00US and are terminated with the proprietary RS-232 connector on the Z50 side. You could always cut the centronics connector side and solder a DB-9 connector.
Since the serial cable is so crtical to the Z50 for default interfacing, the docking cradle package might be a wise investment for those of you missing one.
Good Luck.