We've inherited my husband's 70s cot
....sans mattress. (the old mattress did my husband and his brothers, then the cousins, and has been chucked out now). It's longer and slimmer than most modern cots and neither the mamas and papas nor the Debenhams mattresses come close in size. It also has convexly curved ends, so the mattress wouldn't even be rectangular.
The commonsense me thinks I should just be able to get some foam and cut it to shape...... but what sort of foam are you supposed to use and where to get it from? Or is there a company anyone's used who supply bizzairely shaped mattresses?