There is/was an American Classic Conversion Cassette. I presume you could find one if you hunted. E-Bay?

American Classic

Maybe other brands too.

Looking at the cassette pitch:
https://en.wikibooks.org/wiki/Bicycl...ing_Dimensions

9s Shimano: Pitch: 4.35mm, Sprocket: 1.78mm, Spacer: 2.54mm.
10s Shimano: Pitch: 3.95mm, Sprocket: 1.6mm, Spacer: 2.35mm, Stack: 37.2mm
10s Campagnolo: Pitch: 4.15mm, Sprocket: 1.7mm, Spacer: 2.42mm, Stack 38.8mm
11s Shimano: Stack: 39.0mm

So, the Campagnolo pitch is about 0.2mm wider than Shimano. And, over 5 speeds from the middle, that is 1.0mm (which may or may not work), but over 10 speeds, adds up to about 2.0mm.

This means that you should be pretty close if you found a 10s cassette with 100% loose (or riveted) cogs. For example, Shimano 4700/HG500 cassette.

Then replaced the 10s spacers with 9s (2.54mm) spacers.

Your cassette will be about 2mm wider, and you would be best off using an 11s freehub. If you don't have a 11s freehub, you could mount 9 of 10 sprockets, lock out one sprocket on the derailleur, and whatever spacers you need.

Once you build the first conversion cassette, then just keep the spacers, and the future ones should be easy to configure.

You may have issues with sprockets with built in spacers. It looks like the 4700 cassette above has 2 or 3 sprockets with built in spacers depending on the version. That may not be enough to cause issues. You could probably thin a 9s sprocket to work if needed (11T, 12T, and maybe 13T).