A few days ago the Metropolitan Museum of Art in New York City made their collection more accessible. They've organized it into two categories, that which they know falls into the public domain (which is now free to use for non commercial purposes) and that which may still be under copyright or other restrictions.
They've got a searchable database, where you can click 'Public Domain Artworks' as a search criteria, to be sure you're not finding images you can't use in your project. You must also attribute the work (the Met's website + artist).
I've been to the Met a few times and their collection is incredible. Looking through the images, they've also included works that are in the collection but not on view, which is pretty awesome. There's a search box for artworks on display but not, alas, artworks not on display.
Here's a nativity scene made from limestone by the circle of Antoine Le Moiturier c.1450.