How would you like more than 5 MILLION images from old books to steal?
The Internet Archive Book Images Flickr feed has got you covered. The site seems a bit flaky today, the day I decide to feature it as a resource, of course, but in general it is a fascinating cabinet of curiosities that's free to plunder.
(The Internet Archive itself is a major resource of free materials of all kinds, so even if you're not a visual artist, there may be something you can use over at the main site: books, recordings, movies, and more.)
There's not really a way to search the database for specific images because they're not tagged with that kind of metadata, so it's more of a serendipity kind of site. Keep jumping around until you find a sequence of pages of images that are useful to you.
Pro tip: in the URL line, you can jump to any page by replacing the page number with some random number <52000. For example, https://www.flickr.com/photos/internetarchivebookimages/page12345UPDATE 6/19: The site is still quite unreliable; perhaps it's just my computer (as Yahoo had the nerve to suggest).
For each photo, you can (after signing in) favorite it so you can find it more easily later; share it via the usual suspects; and download it in a variety of sizes.
Check it out with this image:
I'll leave you with some samples: