Remember, you can search for an opera in IUCAT and use the left-side facets menu to limit to video and film.
Search the RISM online catalog to find manuscripts, facsimiles, libretti, and early copies of music, most especially from 1600 to 1850.
Jean Baptiste Lully collection at the University of North Texas
Index to Opera and Ballet Sources
"Opera & Ballet Primary Sources (OBPS) is one of the most recent and ambitious endeavours to make sources related to opera, ballet and other genres of music theatre easily accessible to scholars. The project focuses in particular on performance materials: libretti, ballet scenarios, scores in various formats, mise-en-scène documents, and programme announcements."
Digital Scores and Libretti (Harvard University)
Rare or unique scores and libretti from the collections Harvard's Loeb Music Library, including first and early editions and manuscript copies of music from the 18th-early 20th centuries. As of 17 October 2017, contains over 945 titles.
The Juilliard Manuscript Collection
Digital image collection of 140 autograph manuscripts, sketches, engraver's proofs, and first editions of works by Mozart, Beethoven, Stravinsky, Debussy, Britten, Mahler, and many others.
The Morgan Library & Museum: Music Manuscripts Online
Digital facsimiles of the extensive collection of music manuscripts owned by the Morgan, including works by Bach, Beethoven, Brahms, Chopin, Debussy, Fauré, Haydn, Liszt, Mahler, Massenet, Mendelssohn, Mozart, Puccini, Schubert, Schumann, and many others.
Austrian National Library: Digital Resources, Music Prints & Manuscripts
Digitized scores in the Österreichische Nationalbibliothek (ÖNB). Truncated keyword ("any") search on "musik*"(Alle Felder) limited to Digital (Digitale Ressourcen) refined by combined mediums "Music print" (Musikdruck) and "Music manuscripts" (Musikhandschrift).
MDZ (Münchener DigitalisierungsZentrum) Digitale Bibliothek: Musikalien (Notendrucke) / Scores
Targeted retrieval of over 20,700 digitized scores from the Bayerische Staatsbibliothek (Bavarian State Library). Note the English interface poorly translates this collection as "Music supplies (notes)." For digitized music manuscripts and specifically grouped prints, scroll through the MDZ's themed collections of digitized music scores (print and manuscript), librettos, and books on music. The Munich DigitiZation Center (MDZ) handles the digitization and online publication of the cultural heritage preserved by the Bavarian State Library and other institutions.
Saxon State and University Library Dresden (SLUB): Digital Music Collections of the SLUB
Digitized manuscript and printed scores from the Sächsische Landesbibliothek – Staats- und Universitätsbibliothek Dresden (scroll down for results list of +8,760 entries). Link includes manuscripts ("Strukturtyp: Handschrift") and published scores ("Strukturtyp: Monographie") mixed with textual materials. See also SLUB's Music Collections webpage for links to themed digital collections
Staatsbibliothek zu Berlin - Preußischer Kulturbesitz: Digitalisierte Sammlungen: Musiknoten
Use the left sidebar to explore 12 categories of music scores--including manuscripts ("Musikhandschriften") and imprints ("Musikdrucke")--in the digitized collections of the Berlin State Library.
Gallica: Manuscrits musique
Links to manuscripts with musical notation and also to treatises and materials related to music (composer letters, etc.)
The RIdIM database helps you discover music, dance, theatre and opera iconography images
RIPM (Répertoire international de la presse musicale) provides full-text access periodical literature focused on music and musical life from approximately 1800 to 1950. Includes access to primary source music periodicals dating from the Early Romantic to the Modern Period. Collectively this resource offers both an almost daily chronicle of musical activities extending over a period of 200 years.
The full text of The Times (London), 1785-2019. Does not include The Sunday Times. All articles are displayed as digital page images and all allow full-text searching.
The ANNO of the Austrian National Library offers many digital newspaper and magazine items related to music.
The Themes page of the ANNO offers many areas to explore. Notice that there is a separate tab for Music (Musik)