I've never found a supplier for new cups for Shimano hubs. I have considered replacing with cartridge bearings, but a few things would stop me.
a) where would one find the internal threaded axle sleeve and nut?
b) what about the dust covers?
c) how would we deal with freehub bodies?

In the end, I could never source a complete set of parts for any hubs I had. So it was easier to rebuild wheels with new hubs/spokes which are easily available.