Evidence sources in music usually consist of scores, manuscripts, treatises, and peer or audience criticism (for example: a review of a music performance).
Often, resources like these are "primary sources."
IUCAT
Find books, scores, and recordings held in IU's library collections.
One great way to find evidence sources is by looking for collections of primary sources in IUCAT! The titles of these often include terms like "source readings," "letters," or "a reader." These may be collections of letters by a particular composer, essays written by music theorists and thinkers of a particular time period, or focused on a particular subject like aesthetics. Many of these are music-specific, but you can also search for collections relating to a more general topic, like the Italian Renaissance!
To find these collections:
You can combine a composer name or era (such as the Baroque) with a keyword. Potential keywords include: "source readings", letters, correspondence, and sources. Once you've conducted a search, restrict using the Format tab to Books or E-Books. You can also restrict by language to include only English-language works.
Here are some examples:
DIAMM (Digital Image Archive of Medieval Music)
A portal and for the study of medieval manuscripts. Lists collections and individual sources of medieval polyphonic music dating up to 1550.
Répertoire des manuscrits médiévaux notés
Includes actively maintained lists of digitized medieval music manuscripts grouped by repertoire and location (starts mid-page). Part of the network Musicologie médiéval: Resources for medieval musicology and liturgy. Production of Grégofacsimil, created and administered by Dominique Gatté
Codices Electronici Sangallenses (CESG) = Digital Abbey Library of St. Gallen
Creates a digital, virtual library of the medieval codices (currently numbering 600 and growing) in the Abbey Library (Stiftsbibliothek) of St. Gallen, Switzerland, 19 of which contain musical notation.
e-codices: Virtual Manuscript Library of Switzerland
A continuously updated and expanding virtual library accessing all medieval and a selection of modern manuscripts held by 70 Swiss libraries. As of 30 May 2017, there are 1,749 manuscripts online, 78 of which contain musical notation.
Early Music Online
Digitized collection from holdings at the British Library of over 320 volumes of 16th-century anthologies of printed music (mainly partbooks of vocal polyphony), representing ca. 10,000 musical compositions, published in Italy, Germany, France, and England. Maintained by Royal Holloway, University of London and the British Library.