Check the John Deere part website. they have parts list and drawing of all section of all of their tractor.

http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePageServlet_Alt

Also for a part as simple as the key in an axel, you probably can buy the exact replacement fro any John Deere Dealer. I have never been to one that did not have the part I was looking for.

As a last resort, you may try any auto parts store and find a key that would work.

If you can measure the size you need using calipers most hardware and home centers stock keyway stock.