Check a farm supply store. I’ve seen them for as cheap as $10-15 at farm supply stores out in the boonies. Fleet Farm has some options on their website as well that are a lot more reasonable than the $80 that NRS and Hyde charge: http://www.fleetfarm.com/search/tractor-seats
Not the exact same thing, but might work for you depending on what you’re looking for in a new seat.