Archival records enable you to determine, for example, how frequently Wagner’s operas were performed at the Wiener Staatsoper in a given time period or the salaries paid to Concertgebouw musicians in the 1890s. However, the availability of the archive’s online presence does not ensure the availability of archival records;the bulk of archival records have not been digitized or even indexed.
For many ensembles and venues, the best finding aid may be secondary sources, including histories of the ensemble written using archival materials. Many archives provide a bibliography of publications written using archival documents.The list of literature provided by the Gesellschaft der Musikfreunde in Wien is a good example.
ArchiveGrid is an online database of over 1,000 archives around the world. While not comprehensive, this is currently the best online source for identifying archives.