I can't get enough of illustration. I hope you're getting your fill too, you can read the other posts Illustration and the Illustrations of Edmund Dulac (check it out). You don't have to take my word for it; there are oodles of illustration resources at your fingertips. I've been getting into the public domain for some time now, and it's absolutely filled with images of all kinds for you to use any way you see fit. Of course I have the responsibility to tell you to make sure any images you use, (outside of “fair use, " i. e. - viewing, or teaching about) you have the right to (copyrights, and so forth- depending on where you live).
Now, the internet being a reference source is akin to taking a card catalog and throwing all the cards on the floor for you to sift through. If you're not sure exactly what you're looking for, it's not so easy to find it! Know what I mean? Luckily there are organizations who take the time to categorize and showcase the public domain material for you.
If you're not looking for anything in particular (and have the idle time to browse), a good source is wikipedia. I call it “the people's encyclopedia:" by users, for users. Most of the images they use are already in the PD. If you go to their images in the public domain database you'll find thousands of images in alphabetical order from everything from photographs to maps to illustrations. That's where I ran into Edmund Dulac. So you can find book illustrations and artwork of all kinds in there.
You can just google “illustrations in the public domain" or look in wikipedia if that's what you're looking for. But you have some better options. One of my recent discoveries is a site called Old Book Illustrations. The name says it all, and its loaded with illustrations and most importantly information on them. One of the drawbacks to using wikipedia or other databases is you rarely are given much information on the image in question. You may find a decent picture, but it may not even tell you the author or artist, let alone explain what's going on in the picture.
Old Book Illustrations takes care of the misinformation for you. They have a running blog that seems to keep updated several times a week with a new entries on seemingly random illustrations. The illustrations come from books of all kinds such as old encyclopedias, to science and technology journals (pretty comical way back then), and fiction and non-fiction alike. The blog entries will show the image, and give either an explanation of it or an entry from the original book. I like this one about a Monkey and a Miser, from “La Fontaine's Fables. "
Another great source for books is Project Gutenberg. This database is filled with thousands of books belonging in the public domain available for download in a zip file, or html, or you can just read it on your screen right then and there. I've stumbled across plenty of nicely illustrated classics there with all original illustrations. You can find Aesop's Fables, Alice's Adventures in Wonderland, and whatever else you want to find, I'm sure it's there.
Maybe these resources can help you find some decent illustrations. Happy hunting.
Dan Kretschmer keeps a daily blog at www.vincesear.com