According to the list of works for viola on wikipedia, there are a number of compositions for viola and piano by Zitterbart... He wrote a total of 13 Viola Sonatas, and several smaller pieces for the same instrumentation. Many of them are available here in manuscript form here: https://digital.library.pitt.edu/collection/fidelis-zitterbart-manuscripts. The American Viola Society also published his Sonata number 2 in G-minor (beautifully edited by David M. Bynog in 2015) and his Three Pieces. These are available at the American Viola Project: http://www.americanviolasociety.org

Sheet music for many of Yngve Sköld's pieces can be found for purchase here: https://www.svenskmusik.org/en

Die Manuskripte der Bratschenwerke von Erich Marckhl sind alle hier zu finden: https://phaidra.kug.ac.at/

Scharwenka's piano quintet (op 118) is available now... at imslp: https://imslp.org/wiki/Piano_Quintet,_Op.118_(Scharwenka,_Philipp) and on youtube in a really excellent performance: https://www.youtube.com/watch?v=6_guKodK1JI