DD sleeps on the top bunk. We have an old mattress up there and she is complaining it is "too hard." Actually, I don't blame her b/c when I climb up there to change her sheets it is kind of hard. Grandma sleeps in the bottom bunk when she sleeps over so I can't switch the mattresses. Plus, the mattress on the bottom is very thick (high) so it wouldn't work on the top bunk. It is almost as high as the rails. Having said all that, where should I go to buy a comfortable, yet thin mattress that will work on the top bunk? Is my best bet a mattress store or can I find a comfy one at Ikea, Target, dept. store, etc???? TIA!