Depends on what you want.
I bought a foam & memory foam mattress from 'Memory Foam Warehouse'
for our Hymer drop down bed, got a double and then used the old mattress as a pattern to shape the 'front' of the new mattress to fit.
It took a bit of tailoring to get the shape right and I had to put a few tucks in the supplied cover but it works well.
I got a normal double size at 18cms depth. Ihink it was just under £200.00 which was a lot cheaper than anywhere else I could find at the time, a proper shaped Hymer one is extortionate and we found the old original one was a small double with a lump of foam stuck to it to give the shape!