I went to a boat canvas shop were they also rebuild boat interiors. Gave them the size to cut a piece of 4" foam for the bunks and then added the original 1 1/2" bunk fluff material on top and stuffed both in a cotton zipper up mattress cover. Now its very sleepable.
The Foam Factory is where I buy much of my foam stuff. They are somewhat local to me, but I have picked up and shipped purchases. Two of our children's beds at home have the base/memory foam combos which are great.
A & A parents of DD 2005, DS1 2007, DS2 2009 2011 Suburban 2500 6.0L 3.73 pulling 2011 Heartland North Trail 28BRS 2012 VW Passat TDI
For our bottom bunk (full size) we cut two 3" memory foam mattresses (1 was given to us the other from WalMart) then zipped it all up in a mattress cover. It's much more comfortable now. We've not done the top twin size bunk yet, but it doesn't get used often anyhow.