I'd imagine that like everything else in the trailer there are 2-3 companies that make 99% of the fireplaces. Etrailer has a bunch listed. I've also read (somewhere on the internet) of people going to walmart/home depot and buy a free standing fireplace then removing the out enclosure and installing as needed.
2020 Flagstaff Super Lite 26RBWS
Former: 2017 Rockwood MiniLite 2104S
2015 Silverado 2500HD 6.0L Gas