There are a number of great sources to help you get started on role study research and for exploring opera performance in general. Here is short, curated list of some of the resources most directly relevant to your subject area and available online through the University of Toronto Libraries.
The predominant guide for citing your sources in musicology is the Chicago Manual of Style. The 17th edition is available as of Sep. 1, 2017.
The CMoS can be a challenge to use if you're not familiar with it, so the library created a citation guide with music specific examples throughout. It includes examples of footnotes/endnotes and bibliographic entries, along with links to the CMoS if you'd prefer to read the rules in their entirety. It also includes samples with helpful descriptors identifying the various elements needed for citations, as with the illustrations below:
Example of a footnote:
Example of a bibliographic entry:
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