Replacement Bunk Mattress

Fabric stores may have foam that you need, you may to trim because the bunks in mine have a little angle on one corner

Some of those bunks are custom sizes to fit the bunk so it might be tough to find something in stock. There are mattress mfgs that will make you anything you want. I have a friend that used to order all his mattresses for his house from a local mfg that would make it to your specs.