Standard Ebooks has a database of public domain oil paintings for use as book covers. SE is strict about copyright clearance and requires either scans of a copyright‐expired publication containing the painting, or an explicit statement from a reputable museum that the work is CC0. Here’s an entry I contributed:

https://standardebooks.org/artworks/arthur-i-keller/calvin-c...

To be specific, this is US public domain only (which is globally non-standard).