Great News from RISM!If you research or play music from earlier times, you'll know how important it is to see the sources that our forebears composed, published and played from.
Nowadays, the first problem is locating these materials!
For printed publications, we have Copac (British national and universities combined ('union') catalogue. There's also WorldCat, which pulls together library holdings particularly from the USA and the UK, regardless of public or academic affiliation.
But there's nothing quite like the fabulous RISM, the Online Catalogue of Musical Sources that locates printed and manuscript music from worldwide locations. It was a really useful resource when it was just a series of printed books, but now the free RISM Online Catalog offers online access to series A/I, A/II, and years 1500-1550 of B/I. (See the RISM Publications page to understand precisely what is in the various series!)
Type keywords or a composer into the search box and see what you find - details of the publication, and where copies can be found.