You could try here. https://www.steinertractor.com/
Also John Deere themselves might still be able to get them.
John Deere X758 with a 60" HC MMM deck and a 47" Front mount Blower.
John Deere 17p cart
You also might check:
They have obsolete bearings for autos, trucks, and some industrial/ag.
If they can't help, they can likely refer you to a shop that has them.
They also make custom babbit bearings.
It's a little late in coming but john deere actually had both main bearings in stock along with all required gaskets. Rod bearings were ok. just an fyi for anyone in the same predicament.