Hey Mark — as you know, cable companies don’t offer this off the shelf, but in my experience most any of them that hand build their cables will make a custom build for you. The two brands I deal for, DH Labs and Synergistic Research, certainly will.
The one watch out I would caution is that you will need to absolutely make sure it is wired right. 15A cables typically have their hot and neutral conductors twisted/flipped at one end of the connectors, or else they will be out of phase. If you already have a stock cable of sorts, you can use a multimeter to check for the continuity of each hole in the cable and diagram it for whatever company you want to build it.