painstakingly generated search queries for various websites/databases:

Internet Archive - search public domain works:
    https://archive.org/search.php?query=possible-copyright-status%3A%28NOT_IN_COPYRIGHT%29%20OR%20licenseurl%3A%28%22http%3A%2F%2Fcreativecommons.org%2Fpublicdomain%2Fmark%2F1.0%2F%22%29%20OR%20licenseurl%3A%28%22http%3A%2F%2Fcreativecommons.org%2Fpublicdomain%2Fzero%2F1.0%2F%22%29

    to further customize the query use:

    https://archive.org/advancedsearch.php

Wikimedia Commons - search public domain images:
    filetype:image incategory:"cc-zero|PD-user|CC-PD-Mark|PD-Art (PD-old)|PD-Art (PD-old-100)|PD-Art (PD-old default)|PD-Art (PD-old-100-1923)|PD-Art (PD-old-70-1923)|Library of Congress-no known copyright restrictions|public domain"
