Yes, we do keep foam on hand. We generally have full sheets of popular thicknesses, 50mm, 75mm, 100mm thick, and also various densities, for firm seating or softer foam for backs.
Quick-Dry foam, that allows water to flow through, and Fire-resistant foams are also available.
Special shapes and sizes are possible, and offcut bins have some smaller, cheaper pieces.
Foam “crumbs” are also available (small pieces to fill odd shapes). Some people like to use them instead of the polystyrene balls in bean bags because of a lesser choking risk.