A:Answer From what I've found, the 40mm bands also work for the 38 mm watches. When Apple released the Apple Watch 4, they changed the sizes to 40mm and 44 mm (from 38 mm and 42 mm). Because this change was just on screen size, you can use a band that says it's for a 40 mm watch for the 38 mm. That should hopefully make your search easier, as there's a ton of options for 40mm watches. Good luck!
