Do a search in your area for a foam shop. They will have lots of foam to choose from. Different firmness, thickness, memory foam.....
And they can custom cut it to any size you want.
Alternatively buy a couple of foam mattress toppers and cut them to size yourself. I know as soon as our kids get a little bigger they will need better mattresses on the bunks in our trailer too.
We have used these guys in the past.
They custom cut all the cushions and mattresses in an old used trailer we bought. They are in Calgary but you should have something similar in your neck of the woods.