@jolter
When it comes to repertoire, it’s a difficult thing to puzzle out if you want to start up your own quartet or sextet.
Let’s start with what’s available legally on the market, that I remember off the top of my head.
Ahlberg & Ohlson published music themselves. I don’t have any lead on where to find any of it and I suspect there is no one who can claim the copyright in order to release the music.
The publisher & music store A Th Nilssons Musikhandel, Norrköping published a stream of booklets for sextet, over many decades, 69 of them. Some of them had additional parts for octet. They also did a series for quartet but I don’t know how many issues it had — I only have booklet #2. Each booklet is 15 to 20 tunes in varying styles of the time. They usually start with a medley, then various dances (waltzes, polkas, schottis, one-step, fox-trot etc), a hymn or folk song, and one or two marches per booklet. An excellent cross-section of the popular music of the time.
Booklets #40 through 69 were at some point archived with a composer’s association and are available for purchase from their site:
https://www.svenskmusik.org/sv/sök?page=3&search-text=Album+för+mässings-sextett
Order by emailing their head of archives, Per Floberg. His email is on the web site. They charged me ca 500 SEK per booklet plus postage.
The Swedish Association of Orchestras (SOF) have done the community a great service by cleaning up and securing the publishing rights for a lot of old music. If your Swedish is up to it, you can probably search for quartets (kvartett) or sextets (sextett) and find a lot of rep. The prices are very fair.
https://www.orkester.nu/butiken/
Going outside what is currently on the market, the best bet seems to be to befriend some other enthusiasts. In the days before the Internet, players and groups might exchange photocopies as trades, or players moving to a different town might make a copy of their old band’s binder, if permitted. Or, the binders were sold to other groups when a group disbanded.
I play in one sextet which purchased their binders in such fashion back in the 80’s. The other sextet I play in was founded more recently, on the basis of scans from two other sextets where friends played.
I’ve always had a policy of sharing the old stuff quite freely, though some of it is probably in a legal grey zone. It is impractical to research who each composer and arranger is, especially as many “pop” works from the early 20th century were published under pseudonym. (This is where SOF have excelled by clearing many individual works.)
I have written a few arrangements myself as a last resort, when something was not available off these sources, and I believe this is and has been quite common, too. I’ll happily provide what I’ve arranged, but of course I only own the copyright for the instrumentation, not necessarily the songs themselves and as such can’t be too forthcoming in public.