What is the shortest size cable you can get and where do you find them?
Also, when a pattern calls for a 16" cable, does that include the needles or just the cable?
I have interchangeable knit picks and the shortest cable is 16". Add 2 needles and it’s way over. 
16" needles are point-to-point. I actually don’t think the interchangeables come that small since the needles themselves take up most of the length.
The 16" fixed needles have a shorter tip than the interchangeable tips to give you more leeway. The KnitPicks fixed 16" needles are pretty good and reasonably priced.
The shortest needle I’ve every used was 12" but it was like knitting with toothpicks attached by a cable.:teehee:
Needles are measured tip to tip when attached to the needle.
Most circular needles can’t include a cable short enough to create a 16" due to the length of the needles themselves. Most of the circulars like Knitpicks Options have the 16" sold separately. The very short needles needed to accommodate a short cable can pretty uncomfortable to use.
As an alternative you can learn to do magic loop or use two circulars. I have the 16", but prefer to use magic loop now because you can start a circular project like a hat and finish it w/o having to switch to DPN or another circular method for the decreases.