I imagine that it's similarly difficult to find new typewriter ribbon stock (and for much the same reason), though I confess that I haven't looked.
I find that NOS 5-speed Sanshin freewheels are surprisingly cheap. But these are for touring. I don't suppose that they're so very durable; so if I'm at a bike "flea market", I look around for spares. Some may be hidden among the corncobs that are always to be found on offer. (Different strokes and all that.)
This particular vintage rider lives on flat land in Tokyo, and rides along levees of rivers only (i) when he doesn't have much time or (ii) in order to get to where the rides become interesting. I have what could be called a vintage bike and am very grateful for its 14-to-28 freewheel and its triple crankset: thanks to this combination I've propelled it at least twice from near-sea-level home to above a thousand metres and back home again. (Suntour) gear changing is normally very smooth (though recently the chain has been moving only reluctantly from the 39- to the 50-tooth chainwheel); and if the wide range means that I'm often not in
precisely the gear I wish I were in, I'm tolerably close to it.
You're lucky enough (i) to have "a mint, Campagnolo Record hub" and (ii) to live in a nation where a certain subculture of grandpas drools over and pays a premium for Campagnolo Record. So sell the hubs, buy replacements from Zeus, Suzue, Sanshin or wherever, and with the change, get the freewheels that you want. They're certainly available:
as well as from so-called flea-markets (a misnomer; most are very hygienic) and I imagine also elsewhere in Japan: if you get talking to other people riding old bikes they may well have good suggestions.