not sure if this is something they would stock but the deepest catalog of obscure (as well as commonplace) decals seems to be held by
Screen Specialty Shop (known as sssink)
https://sssink.com/sssink_2010_016.htm

however they USED to only sell decals to verified professional refinishers, however I've heard tales that suggest they have relaxed that practice (maybe) and some regular citizens have purchased decals from them.
YMMV
That above link is for custom work, but this is to their online catalog (which may not be the full range) and a quick scan does not show eiither "BMW" or "Montague"...but worth asking them
https://sssink.com/bicycle-restoration