I see I have been beaten to it. in the post above.... but today in Aldi an inflatable tots bed - bit like a rectangular paddling pool in similar material to inflatable mattresses.. says 'kit' on the label so I am not sure what else it might include. It would take up a b it of space and definitely not fit on a single bed. When ours were little I bought a suitable sized rectangular piece of tent window mesh, bound the edges, added poppers, so when clipped in place to the edge of the bunk and the bottom edge of the bunk above prevented any fall-out!
Good luck!