I've just replaced the foam in my V berth with this stuff :
The Foam Factory, Mattresses, Conventional Foam
I used the Lux High Quality 2.8lb/sq.ft, 50 lb foam. What I did was buy a king size mattress, which is just enough to do the whole berth and the filler, with some strange shaped bits left over!
At $164 incl. shipping for a 4" king size I made quite a saving over what my local foam supplier would have charged (and did charge, for the salon cushions)
I had read previous threads on buying a Walmart mattress and cutting that up but wasn't impressed by the quality of foam they use.
This seems to be the top quality, high density stuff.
Cut it up with an electric carving knife, $20 from Amazon.