A generation of researchers and librarians will have grown up with COPAC (Consortium of Online Public Access Catalogues), the union catalogue for research libraries in the UK and Ireland.
Today (31 July 2019), COPAC was retired and replaced by JISC Library Hub Discover (https://discover.libraryhub.jisc.ac.uk/). Time to update those bookmarks!
Library Hub Discover currently contains over 39 million records contributed by 110 institutions across the UK and Ireland. The holdings will include books, journal titles, grey literature, etc. but also maps which you can locate using Advanced Search.
Library Hub Discover is a great resource to discover rare and specialist material tucked away in other libraries or to find out what is available in a library near you.
Contributing libraries include national libraries (e.g. British Library, National Library of Scotland, etc.), Higher Education libraries and some specialist libraries (e.g. Historic England Library, National Gallery Library, National Portrait Gallery Library, National Trust Libraries, Royal Asiatic Society, Royal College of Physicians of London, Royal Society Library, Science Museum Library, The National Archives Library, V&A Libraries, Wellcome Library and many more).
While you are here…
… have you considered using WorldCat, a major union catalogue for US libraries? Excellent to just find out whether something actually exists. Access our subscription (better) version or the free version.
… do you know about the Karlsruhe Virtual Catalog, a huge union catalogue based in Germany? Excellent of course for German-speaking countries, but you can also, in a single search, locate collections in worldwide library or library-consortia catalogues.