I've been looking at the same issue/question. At one point I was looking for a sofa bed/bench convertible, but not anymore. I'm building out my E350 with RB50 style layout.
Here is my build thread for background: viewtopic.php?f=24&t=15938
I've also posted a similar question on the interior forum, there are few replies that may help you: viewtopic.php?f=40&t=16410
My plans at this point is to find a takeout seat from an SUV or van that would work for my intentions. My van has a permanent raised bed in the back so I don't have room for a convertible sofa/bed. I had been hopeful I'd have room for it but I found now that it won't fit. So for my needs I'm looking for a 3 person bench seat that folds forward that will provide access to storage under the bed platform behind the seat. Right now I'm planning to use the 2nd row seat from a Yukon, Denali or Suburban. Ideally it would be a 60/40 seat with integral shoulder belts, headrests, etc. I'm not certain I'll get all that but that's what I'm looking for.
Here is a link to one on ebay that may work for my needs:
http://www.ebay.com/itm/2007-2012-OEM-C ... 1155507060
What about ordering an RB50 bench seat straight from Sportsmobile? Do they sell those?
If you do find a source for the euro style rock and roll beds please do post back here.
Here is one other link I found during my search that may help. Someone put a rock and roll seat in a sprinter:
Good luck with your search and let us know what you end up doing.