When an OEM decides to service sub-assemblies as a unit (shaft assembly) instead of individual component items (U-joint), it often means that the constituent parts are proprietary to that OEM and that application.
If your immediate locale has a specialty "Driveshaft Shop" it might be possible to obtain a replacement U-joint there.
They will need the shaft with an intact U-joint to measure in order to determine what will fit.
In this case, the shaft with one U-joint still in place, and the defective U-joint removed (but take the cups and trunnion with you too) will make it simple for them to measure. Good luck!