You can buy so many varied mattress sizes these days, especially on the interwebby, from the standard UK sizes we all know to various continental ones. I think even Ikea have their own sizes, so it might not be too difficult to find one that suits the van, rather than the other way round. Mattesses for bunk beds are usually much narrower than normal beds.
On the other hand, you can easily buy mattress-depth block foam or memory foam and cut it (or have it cut) to whatever size you need. It would need covering though (I mean aside from bedding) you know how foam always feels like it's sticking to everything it touches ! If you can't turn your hand to covers, how about buying a couple of stretchy, single fitted sheets ... fit one from the top and the other from underneath (or a huge one that would stretch right over the whole thing), then stitch everything in place as a permanent mattress cover.
Of course, if you want them properly upholstered as seating during the day, you'd also probably find that most motorhome/caravan re-upholsterers could make something up to fit your sizes.