Better bike shops used to keep a cabinet stocked with Campagnolo spare parts. It didn't make sense to carry spare parts for any other brand. (aside from semi consumables like axles, cones, freewheel cogs, etc)

Shimano was unit replacement only from the very start. If a jockey cage would have broken BITD the usual solution was a new derailleur. The only other option is finding a pulled one in a junk box somewhere, as previously noted.