Once a costly subscription based resource, the RISM (Series A/II) database is now freely available on the web and is linked under both the MUSA and MUSC Subject Gateways. While the bulk of the details for each entry are in German, there is a toggle link to read the descriptions about the project and other features in English. Search by composer name and find out the location of a manuscript by your favorite composer (after the year 1600 AD), then look at an incipit (or beginning musical motive) to help you identify the work you are researching. Click on the link for the owning library and jump to their website to see if ordering scans of the original manuscripts are a possibility. Use the facets on the left side of the result screen to limit results by composer, date, and other options.