AFAIK, all wet baths are custom-molded for the application. While there may be only a handful of companies molding them, each RV manufacturer is supplying either dimensions, templates, or molds.
To buy one ready-made, you'd have to settle for one of the designs already in the market, and persuade either the RV maker or the fabricator to sell it to you, or find one at a surplus dealer.
On this last possibility, I would check with the surplus outfits in Eugene, OR and Elkhart, IN (several listed in the archives here, or a Google search will turn them up). With several manufacturers going belly-up over the last year, you might just get lucky.
Remember, stock wet baths are tiny. Typical applications are truck campers, class-B's, and small light-weight travel trailers.
Good luck with your quest.