I visit a very good furniture reupholster in my area. Family business, been around +75 years. They actually will recover motorcycle seats (mostly HD and very expensive). When ever I need seat foam, I pay them a visit. A few months ago, needed a pice to replace crumbling stuff in a race car seat. They had just the thickness and density I wanted. Cut me off just the right amount, “cash sale”. Look around your area, you might find a similar place?