Build one! You could do it relatively easily! I got a bunch of thick 5-bar (although the thinner gauge was available) from some mill down in SoCal. I can dig up the name but I'm sure you could source it even closer. It's not as hard to find a supplier as I had thought. I was replacing all the Masonite bases for my jump seats. I even gave them exact cut specs and my bases arrived in perfect, ready to paint pieces. Reasonably too. $125 cut and shipped.
I have been toying with the idea of doing a 5-bar tub as well. Mine is rhino'd and I'm not fond of it. I would think that with careful measurements, one could source the three necessary sheets cut to spec, and then have a local shop do the two 90 degree bends for the wells. Of course, if you're looking for a single piece 5-bar bedliner disregard everything I just said! ;-)