In case having Campy part numbers might help in your searches:
On a full-campy bike, that piece of housing from chainstay to mech (with the proper ferrules at each end) would be part number 617 if the bike has a #621 "diver's helmet" chainstay braze-on.
For bikes with no braze-on, they made the housing stop clip #636, and it doesn't use a step-down ferrule. The housing for that came with matching "plain" ferrules at both ends, part #622
This is from Catalog 17a ('79 I think).
I don't remember ever seeing a part number for just the ferrules. I wouldn't be surprised if they did sell bags of ferrules to distributors or bike shops, but they didn't call them out in the catalog, at least not in the NR-SR era or earlier. I stopped paying attention to Campy after Record-C came out.
Oh wait here it is, in Catalog 18 ('84 I think). Plain ferrule is #619, step-down is #620. Sorry for writing the above out of ignorance, but I'm not going to delete it now, after all that typing.
Only the pickiest concours judge would take off points for not having the period-correct ferrules, but if you want it to look excruciatingly correct, please avoid anything made of plastic, or black or any color besides "silver" (broadly defined). Campy's were dull-plated steel, but a silver aluminum ferrule could be acceptable, at least it wouldn't make me want to hurl. They're thicker-wall though, so they don't look completely right to us old guys who grew up with
tutto Campagnolo. Please for God's sake don't use one that says Jagwire or <shudder>
Shimano on it...
Kidding! Well, mostly. I do have a small stash of actual Campy-brand cables, housings and ferrules that I keep for total period-correct builds. But for most bikes, just making it work is good enough.