These suggestions have been made by German and Italian music librarians, members of IAML (The International Association of Music Libraries):-
Goethe Universitat (University of Frankfurt) digitised music, film, theatre resources - including librettos.
This library has large collection of libretti (about 3000) from about 1750 to 1930. Digitizing them slowly - now up to now 458 libretti. You can find them on the repository website http://publikationen.ub.uni-frankfurt.de/solrsearch/index/search/searchtype/collection/id/16108Raccolta Drammatica - digitised libretti, Biblioteca Nazionale Braidense Digitised libretti - about 10,000 libretti.
Also, in the digitized collection of music ("Noten") there are some complete opera scores e.g. manuscript of "Don Giovanni" by W. A. Mozart or a print of "La Vestale" by Spontini.
Another interesting link is the Italian union catalogue (Il Catalogo online del Servizio Bibliotecario Nazionale, or OPAC SBN), which lets you search lots of library catalogues at once.