Yes, Kodak and some other companies from English speaking countries called them 'Film Sheaths', in Germany companies like Agfa and Orwo called them 'Filmhalter'. But when you search for them on places like eBay, try different searchwords as well, like 'film adapters' or 'film inserts'. They can also be hidden under the keyword 'film holders' as they can be called that, even though that usually means holders/cassettes that are used with film only and not adapters. The German word 'Filmhalter' may be automatically translated into 'film holder(s)' on eBay.
Do a search like this on eBay, and tick the box "Include description": (film sheaths, filmsheaths, film adapter, filmadapter, film inserts, filminserts, filmhalter) and you have included most of the possible searchwords. The parentheses and commas ensure that the site will search for all off those individual searchwords. Some sellers don't mention the size, so you'll have to look at the pictures or ask the seller.