The Selectric II ribbons that I saw weren't like the inked-cloth used by most typewriters and dot-matrix printers -- which could be reused a few times, or even re-inked with a special tool. The Selectric II ribbons were more like a plastic film with a black coating that was crisply and completely transferred to the paper where the typeball struck it. (Not great for OPSEC, incidentally.)
The carbon film you're referring to is likely part #1299095 (black correctable). Believe generics of these are still made. I also buy NOS sometimes. So far the 30-40yo cartridges seem to work fine once you get to the spooled film vs the few inches that may have been exposed to the elements. Not sure if the chemistry of them is just that stable (vs ink ribbons) or I've simply had good luck (i.e. I'm not speaking from 40 years of experience or anything).
The tougher one to source is the "bicycle" style of cartridge used on some Selectric IIIs (other IIIs were built/swapped to work with the old II-style carts).
A small wholesaler and retailer I worked for as a kid in the late-1980s did a lot of business in replacement printer ribbons, and there was one third-party brand that offered compatibles for most every printer. Today, I wonder how the manufacturing worked: whether they just sourced from factories that already had the tooling and know-how to make those particular compatibles, or they had to arrange the reverse-engineering and tooling. And whether the same parties still make the ribbons today.
Kind of like Slide Rule Universe: https://www.sphere.bc.ca/test/sruniverse.html