There are many manufacturers of armatures. An old list from Cymbacavum here: https://www.google.com/amp/s/cymbac...ompanies-make-balanced-armature-speakers/amp/ Knowles and Sonion are the most famous; they do both off the shelf and custom versions. But Bellsing is used a lot as well. Molex/Final Audio and Sony make their own. Yashima Electric made the armature driver in the Grado and Ortofon iems if I recall correctly. Of course many of these other manufacturers are lesser known and aren’t in as many brands and models.