A common supplier for typewriters in general is Ribbons Unlimited: https://www.ribbonsunlimited.com/

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).