How about a futon mattress? Not sure what the size of those run though.

I bought some absolutely beautiful horsey type fabric off of an Ebay seller's store. Go there and type in western upholstrey and then check in all the seller's stores.

If you have an upholstery shop close by, take your measurement's and ask them to cut the foam for the cushion (s).  The upholstery shop should have foam.

THEN, go to a shop that sell's material, purchase quilt batting, the thick kind.  You will wrap the foam cushion in the batting.  (A spray adhesive on the foam make's this much easier.  You don't want the batting moving around. You can find this at most any discount or craft store.) The batting is what make's your cushion soft and squishy.  You'll want to cover all of this with a cover.  You don't have to make it removable.  Now, your cushion (s) are ready to be covered in upholstery fabric.

Wrapping the foam in batting is an extra step, but you'll be much happier with your result's if you do this.

Good luck with your project!