One of our most useful resources here in the Crafts Dept. is an enormous collection of clip art books. For those that don't know, clip-art is basically copyright free imagery, and takes many many forms. I think of our clip art collection as a vast library of visual material and use it constantly as reference and inspiration, it's also a great way to learn because there's so much detail in every book. We have lots of books of historical engravings and illustration, books on lettering and calligraphy, all sorts of flowers and plants, many types of textile design, etc. The range of styles is so broad that you can always find a useful reference for any project- below are just a few examples.
Oh how I love Dover Publications. What an amazing breadth of material... I swear I never met a Dover book I didn't love.
The Pepin Press also publishes great clip-art and includes CD-Roms with hi-res images of everything. We also buy Japanese Clip-art books - which can be hard to find- from Kinokiniya, a much beloved Japanese book store here in New York.
And finally, one of my favorite things that we do here is create our own downloadable art for crafts projects to make available to our our readers. That's all archived here, and free, so take a look!