i started in the auto business with my uncle in, i think, 1965 doing brake jobs.
you know, pulling stuff apart, blowing the asbestos all over the place with an air hose.( i love the smell to this day)
linings then were riveted so i had to press out all the old rivets, grab the lining off the shelf, rivet it back on and then arc the linings to get them to some semblance of fitting the re-machined drums.
i rebuilt hundreds & hundreds of wheel cylinders then and it seemed that all the major brake parts mfrs had available (as did old school parts houses) assortments of many different sizes of brake cylinders rubber parts.
i know that this is/was a LONG time ago but i can't help but think that this stuff is still available et obviously harder to find.