Closed cell foam cushions
You might try http://knoxfoam.com where they have miscellaneous size sheets of closed cell foam. You can also buy goods cut from rolls for smaller quantities. I have purchased from them several times. I notice they have 3/4" thick sheets in yellow right now at $40 each including shipping, I usually glue up several sheets of different thicknesses to get the total thickness I want.
Usual disclaimer applies, I dont know these people from Adam, just have occasison to buy their foam for my boat projects and products.