I've been searching and only finding stuff from 2007, 2013... Nothing recent.
However I've found several vendors selling the offset to attach the bed extender brackets, but not the brackets themselves.
I'm starting to think I'll make my own brackets.
Nissan Frontier Tailgate full assembly for sale. Includes factory backup camera. Midnight blue color. This tailgate has been in storage since the truck was purchased in 2015. It looks brand new. $250 or make me an offer.
factory sliding bed extender for Utili-track system $50