Not to long ago I added a greater explanation of the foam choice question to the forum which you might find valuable. It is here: http://www.airforums.com/forums/f46/...ml#post1640704
As for foam providers that will ship what you want and for a reasonable price, try United Pillow in Miami: Foam & Filling: United Pillow Manufacturing
I have been very satisfied with every purchase from them, but mine which have all been variations on square or rectangular and not mattress sizes. I believe they do custom cushions which might require you to provide a template or detailed measurements for your specific curved applications. They will also sew matching white cotton covers with zippers that match your foam order economically if you don't also have them do the final fabric. Two covers encasing your foam will add life to your exterior cushion fabric because the batting won't be abrading the underside of the outer fabric each time you sit down. Of course, this mostly applies to those who spent 100+ days/year using their coach.
Being a big guy (200+), I used their 4518 foam for our dinette bottom cushions. Most medium-sized people (150-185) will like the 4015 foam; smallish people (under 140) will like the 3618 foam for longer sitting sessions. These foams are all long life foam choices used for primary sitting areas. Go softer for the back cushions. If dual purposing the sitting cushions for sleeping, the medium density foams are a good compromise for comfort.