Two sources.
Sailrite does have a cushion foam that is "marine-like". You can read about it on their website. Select 'Online Shopping' and then 'Foam' on the left hand side.My sewing bible, 'The Big Book of Boat Canvas' likes a closed-cell foam called Airex. If you make stuff for your boat it is a good book for your library. Author is Karen Lipe.Be prepared to shop, foam is expensive.