My answer pertains to replacing the top mattress. It is an odd size twin and I had trouble finding something. I ended up getting something from Walmart (site to store pick up) that was "suitable". More than I wanted to pay; it was some sort of dense woven poly thing. I am sleeping on it at 140 lbs. and have for two years. Will it last forever...nope. I may have even cut it to fit...don't remember that part. I sleep on it for 3 -4 months per year; hubby is long so he needs the WHOLE lower corner bed. I added a memory foam on topper 3".
http://www.walmart.com/ip/Thermo-Bonded-Polyester-Core-Mattress-Twin/10902293
