MDS .. I appreciate your response. We bought the receiver factory-new from J&R Electronics and there was no cable.
However, thanks to Gooooogle, I found the cable this morning and bought it from OneCall. For all those out there with the same questions about this 3.5 mm / 1/8" trigger cable, it just so happens that Niles offers it as an accessory part. Their part number is FG00933.
If you search Google for this number you'll find a few online stores who offer it. It's a 10ft mono mini-cable and sells for less than $10. It's a bit expensive, but where I live it's worth it. I was searching for a 3.5mm triggle cable and that got me nowhere. Try searching for the part number or '3.5mm mono to 3.5mm mono' and you'll find it.
Problem solved.