The basic PC sychro cord is the flat two-prong "household" type. The Sunpak cord has a PC male at one end, and two-prong plus round safety prong at the other. The catalog number I have is 651-753. I have no idea if that is still the number, or if they still offer the cord. Any flat two-prong "household" type by PC male cord will work.
Looking in the manual, which you can find on Butkus.org, it's in the Major Accessories section, page 34, item 6. Also pictured on page 3, item 19.
EDIT: KEH has at least two of the correct cords, 651-753, and cheap. They also have several of the genuine Sunpak optical slaves, also cheap. Look under Sunpak Flash, then flash accessories.