W got our 5cm memory foam 1 from Ikea
https://www.ikea.com/au/en/p/tustna-mattress-pad-white-10298217/. It was originally a queen for our Swan, but had to simply cut it down to double bed size when we bought the Expanda. I'm also thinking of "downgrading" from the inner spring mattresses to the foam ones, as we used to a Tempur memory foam mattress at home.
As per my recent thread here, have you any issues with your front fold out bed being lower than the front boot section? Really frustrates me and I'm thinking I might just get a piece of Styrofoam cut to size to make up for the difference in levels.