I think you have very few options. Deere's blade is the obvious choice, although it's not cheap. It is designed for the tractor and fits. Another option may be Bercomac. You can check them out here:
http://www.bercomac.com/produits_lames_pourTracteur_ang.php
Not sure how they compare in quality or price to John Deere.
http://www.bercomac.com/produits_lames_pourTracteur_ang.php
Not sure how they compare in quality or price to John Deere.