Your next step -- look for your score, libretto, the play or other literary source material, historical information, as well as videos and sound recordings. Electronic/streaming as well as print, DVD, CD, etc.
If you can't locate your score via the library catalog, Music Online, nor IMSLP, this collection of library catalogs worldwide will help identify a score you might be able to borrow from another library (using interlibrary loan).
Looking for magazine or journal articles, reviews of operas, plays, literary works...? The following resources will be helpful.
Contains hundreds of reference books including a dozen music-specific ones that are NOT in Oxford Music Online: A Dictionary of Opera Characters, Grove Book of Operas, Encyclopedia of Popular Music, etc. These eBooks are linked via the library catalog, as well as being available here.
Use the form to request articles, books, book chapters, dissertations, or other items that you need. Please check the A-Z Journal list, OneSearch, and/or the library catalog first to ensure we do not already have the item.