It depends if you are wanting to restore to original or wanting a seat to sit on.

http://www.ebay.com.au/itm/NEW-SEAT-FOR-RIDE-ON-MOWERS-FITS-SELECTED-COX-AND-ROVER-MODELS-/181152546291?hash=item2a2d887df3:g:OYYAAOS

This is for some of the Cox, bit on the exxy side but you can get new tractor seats for about $80 if you look around ebay