You can get form cut to size on ebay and some shops or try TVS They have lots of spares and even if you can't see it on the web site I would give them a ring they are very helpfull
Whatever you get (foam, memory foam, airbed) make sire you have some sort of anti-condensation layer underneath. The original Conway mattresses have a sort of plasticky woven layer on the underside of the cover to assist with this. Either reuse this cover, place the new mattress on top of the old one or use something like the underlayer for laminate floorboards under the mattress.
And remember if you make the mattress layer too thick the bedboard wings won't fold shut properly. (Not done it myself, but I've seen it.)
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