I have found a good reasonably priced source of shielded cable for stepper motors and limit switches.  I grounded one end only to the control box.  DIYHomeTheaterSupply.com has shielded 16/2, 18/4, and 18/2 cable in 1 ft increments (10ft minimum) or 50 ft increments. Go to www.diyhometheatersupply.com and click on Shielded and Direct Burial Wire Sold by the Foot in 1ft or 50ft Increments (Specialty Cable) under Home Theater Accessories in the left hand column.  The cable is of excellent quality.

 http://www.diyhometheatersupply.com/Shielded-and-Direct-Burial-Wire-Sold-by-the-Foot-1ft-50ft-s/68.htm