Welcome to Beoworld !
You can buy service manuals from B&O, most are quite costly and they may not even have the BM1900 manual available anylonger.
If a used manual will do, you can do a search on f.e. Ebay, they do come up occasionally.
Finally, if a scan/email is enough to cover your needs why not sign up for silver membership. This will also give you access to other documentations, catalogues etc.
Service manuals don't become cheaper as the items age, quite the opposite actually.
Stating your email address in public is the best way to get lots of spam. I have removed it from your post.
Martin (Beoworld Moderator)