A&A will have them. Like Locomotion said, they're mostly found in '78-up 904s but not all units have them. The info below is cut and pasted from this thread;
Differences in 904 sun shells
Something to be aware of is the later ('78-up) low gear sun shells are different than earlier models. You have to get
everything that goes with it to make it work - the planetary set, the shell, sun gear, snap rings and bearings, etc.
The low gear shell is on the left. See the step at the bottom? The step accommodates a spacer used with the 'low gear' planetary. You have to have that piece or it won't work.
View attachment 1716206726
I believe
all 904 sun gears are the same except the low gear one uses this machined spacer. I don't know why they went with this setup, maybe has to do with the lockup feature on the later units. The lockup units have different input shafts and I think the overall stacking tolerances might be slightly different. Either way, you can definitely use the low gears in an earlier unit but again, you need all the parts.
View attachment 1716206727
