Most major galleries and museums provide searchable access to catalogues and databases of their collections. Search for the title of the gallery or museum and look for links to their catalogues/databases.
The Met has a permanent collection containing more than two million works of art, including works of from classical antiquity and Ancient Egypt, paintings and sculptures from nearly all the European masters.
The New York Public Library possesses one of the largest and finest collections of medieval and Renaissance illuminated manuscripts in North America, yet its manuscript holdings are scarcely known to scholars, much less to a wide public.
The St Chad Gospels (also known as the Lichfield Gospels, the Book of Chad, the St. Teilo Gospels, and numerous variations on these) is an eighth century Gospel Book housed in Lichfield Cathedral. "Turn" the pages here!!
Example of early Islamic calligraphy.