Actually I think the original way was not step-down ferrules, just regular single-diameter ferrules but much smaller. Maybe not made anymore (I'm way out of touch with the current offerings) but I have some ancient Simplex housing and ferrules and they're tiny by modern standards.
One reasonable way to git 'er done is to use regular spiral-wound housing with no ferrule, just grind the ends nice and flat and de-burr the holes. Maybe not as finished a look as a proper ferrule, but functionally just fine as long as the diameter matches up so the housing is guided in straight.

Mark B