I too have been told that they aren't made by any U.S. manufacturer. I read on a boaters forum that some had them shipped from Europe or Australia so that they could be installed on their boats.
nüCamp RV which makes teardrops and truck campers has one they use in a truck camper and has them imported from Europe.
Cirrus Camper’s Fold Out Bathroom Sink
I wonder if you contacted them if they'd sell one to you? Probably not but it might not hurt to try. Or perhaps they could at least give you some pointers on the easiest way to have one imported?
nüCamp - cirrus
661 Belden Pkwy, Sugarcreek, OH 44681
(330) 852-4811