Most dealers have a few extra drive shafts on the shelf for customers who want a newer header on the older combine or putting older header on newer combine. You might find a used set a a good price.
Go to your Deere dealer. They should have the "short" shafts to put on where the PTO style shafts are now. It will be your lowest cost alternative, then when you go to a 50 Series, all you'll need is the "short" PTO style shafts.