UBUWEB tweeted the link to a rip of the vinyl recording of the soundtrack to Tarkovsky's Solaris last night. I haven't really tried to navigate UBUWEB's apparently vast pataphysical archive, but the occasional Twitter update makes for some good passive scores like this one.
You can also look at visuals and listen to a stream of the whole thing at the bottom of this page from the Archive of Andrei Tarkovsky.
Apparently this was recorded in 1972, but the site authors can't find any evidence it was actually ever put out in the USSR. This, despite the fact that the Japanese releases' cover art claims the music is remastered from the original Russian vinyl. I originally went after this link because I wanted to see the original Russian artwork, but alas. The English language art is pretty badass, though.