I don't mean to be a smart aleck, I thought mine came with a cable and was looking through the manual for the list of accessories. I found this on Page 23 of the manual:
"Use a cable tipped at each end with a 1/8-inch (3.5mm) mono phone plug to connect the Model 990’s DC TRIGGER OUT 1 jack to the amplifier’s DC Trigger Input jack."
I hope that helps.