[gmcharlt]

Many libraries [1], including the likes of the Library of Congress and the National Library of Australia, make their catalogs' MARC records freely available via a library-specific protocol called Z39.50. The Library of Congress makes their catalog metadata available in other ways [2][3] and the Internet Archive has a collection of MARC records as well [4]. There are also a couple commercial services that provide them, and publishers, particularly of digital collections used by libraries, will sometimes supply MARC records (though they tend to be low-quality).

[1] https://irspy.indexdata.com/ [2] https://www.loc.gov/cds/products/marcDist.php [3] https://id.loc.gov/ [4] https://archive.org/details/ol_data