A lot of stores like Cabela’s and REI have cushions meant for hunters that are about that size and shape. My first instinct, though, is to look at a department store.
As an alternative (or while waiting for the Amazon delivery) a quilt or wool blanket folded until it’s about 5”-6” thick makes a great cushion—just the right compromise between stability and give. It also lets you customize the thickness should you find you need more or less height.