OK, this is mildly interesting. I was doing research and ran across the Internet Music Score Library Project (IMSLP).
I noticed, in the categories, a section breaking down all the music by the instrument featured. Clicking on the "trombone" category I found two pieces...free for download..
Rimsky Korsakov, Concerto
Biduo d'ouro, a trombone/tuba duet by Zoltan Paulinyi
(Can't say that I know that second piece.)
The RK was uploaded by Lyle Neff. I haven't met him but I'm guessing he's either a librarian at the University of Delaware. or a hotheaded Canadian nationalist.
(and, just in case, Long Live Canada!)