For the wet bath shower pan combined with stool mount and blackwater tank, called rv shower pan combo, you can go online and do a search for them. Here are three:
www.rvpartsnation.com
www.pellandent.com
www.ameri-cart.com and there are others.
Each company will have multiple units at varying prices. Don't pay the highest as they're all pretty much the same quality. The first place had one for $159 or there abouts and the others had some higher by increments of $10 or so. When I started looking they were like $450 or more. Taking a step back and waiting a while I got mine for the above lower prices. Worked perfect. You just need to be exact where you want your 3"black water drain located in the bottom of the tank under the stool and where you want the shower pan drain located and exact measurements for your available space/dimentions, length, width and height. You will need to add some support under the shower pan side. If you don't installment a grey tank for kitchen sink, shower and bathroom sink be REAL sure you open your small drain line BEFORE running water down the drain. Found out the hard way just how fast that skinny drain backs up and into the shower pan! You'll need room under the shower pan for a pee trap to prevent unwanted odors from that drain. The stool is a direct dump and the bowl knife does the job in the black water tank.....along with chemical deodorant. It's aprocess for sure.