As for me, finding step-down ferrules was once a very difficult task. Recently, I found a shop here in Sendai that has a jar of metal ferrules. Apparently the proprietor obtains them from Shimano.
MORE: recently I visited another shop owned by a friend who is both the proprietor and mechanic. I needed some advice concerning a derailleur hanger. He spotted that a plastic ferrule contained in a Shimano cable kit was disintegrating. He scolded me for using it. The metal variety was substituted with satisfying results — not a perfect fit for an ancient Sugio 75 component, but good enough.
AND: more recently I've learned that some vintage components do not mate exactly with currently available step-down ferrules. At least not the ones I can find. I can't exactly set down the details here. So if this dinosaur thread stays open for added discussion, contributions and blah blah details, I'll be a faithful follower.
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Vitus 979, Simplon 4 Star, Gazelle Champion Mondial, Woodrup Giro, Dawes Atlantis