Found similar mattress at WalMart and Sears websites but they were both out of the twin size.  Ran across the Cool Gell mattress at Wayfair and figured we would give it a try.  The sofa bed mattress arrived two days ago.  After cutting open the plastic wrapper and unrolling the mattress it took several hours for the foam to fill with air.  Once it did it was as advertised in thickness.....very comfortable.   This mattress was a few inches longer than the old one but it still folded up nicely with a few seconds extra effort.  I would definitely recommend this mattress.