wow...that's a toughie.
I'd say a google search is always the best way to start with what you're looking for. Image search certain criteria (like the Musketeers), check a few images, and if you find stuff that you're looking for, check the site of the place that hosts the image. See if they have more material, see if they were using books, see if they recommend books. Then it's off to a library for researching more images. Heck, doing something like you want, it wouldn't hurt to find books with images and text either.
It's a pretty painless process (unless you need obscure reference material), and what I've been doing for most of my art classes.
Bookmarks