I know your request was for art prints but I did want to tell
you about a resource that we all have for excellent photos. I
primarily have used it for animal photos, since they are a
subject I ask students to use every year. A few years ago a family
who owns an advertising agency offered me their old photo stock catalogs.
They are full of excellent photographs of many subjects (some not
appropriate). I simply had some students remove the photos I wanted and
I keep them in a folder. It is so much easier for a child to
look at a photo of an elephant than to draw from memory.