The following resources were found or supplied from various websites.

Claudio Abbado - Digitized Scores

Maurice Abravanel - Digitized Mahler Scores

LIBRARY OF CONGRESS - The Music Division has digitized many, many scores in our collections. They can be accessed in several different collections, all of which are listed here: https://www.loc.gov/collections/?fa=partof:music+division. There are digital collections focused on sheet music for piano/voice, either on specific topics like baseball music, or chronologically, with collections devoted to segments of the 19th century. There are also digital collections like the Music Treasures Consortium ( https://www.loc.gov/collections/music-treasures-consortium/about-this-collection/ ), which focus on manuscripts of major composers that are in our collections.