Pretty much all manual typewriters use 1/2" (12.7mm) wide ribbon which is most of what you're probably going to find in the marketplace.

The thing that changes from machine to machine is is the potentially proprietary spools and those are usually specific to how the ribbion auto switch is effectuated. Most ribbon comes with small grommets about 10-12 inches from the ends as many machines need this to trigger the switch over. If the plastic spools you purchase don't work with your particular machine you simply spend a minute or two to hand wind it onto your existing original (metal) spools and go from there.

There are lots of videos on YouTube showing how to hand wind ribbon onto a machine. Sarah has a pretty reasonable one: https://www.youtube.com/watch?v=up412FjTEkw

Even Tom Hanks has a ribbon changing video... https://www.youtube.com/watch?v=GBbsNKaVAB0

Incidentally, if your seller specifies them, the Underwoods take Group 9 (GR9) spools. Likely not helpful or illustrative for you, but certainly interesting from a historical perspective, Ted Munk has a catalog of Typewriter Ribbon varieties Offered by Underwood in 1956: https://munk.org/typecast/2020/08/23/typewriter-ribbon-varieties-offered-by-underwood-in-1956/

reply to u/prettiestGOAT at https://old.reddit.com/r/typewriters/comments/1g8z0fm/can_anyone_help_id_this_underwood_typewriter_and/