Ray and Les, are you talking about the “flex drum sanders” the OP was referring to or the pneumatic drum sander? I am going to assume you are talking about the pneumatic one since the flex ones are not inflated - they are simply a dense foam core with sandpaper wrapped on them.
Regarding the pneumatic ones, the ones that I am aware of have a “rubber type” bladder that you can add air to. The ones I have are small ones, like these for the Foredom from Judy Gale Roberts site: https://intarsia.com/collections/hardware/products/kirges-inflatable-sanders?variant=27764085830 and the Guinevere Sanding ones.
if you go to the link at Judy’s site, you will see they come with a small hand pump. She also provides a link to this video from King Arthur’s Tools and their Guinevere setup and how to fill them.
I don’t (yet )have a bigger pneumatic sander, but believe they are filled similarly.