I just did a google search for "free clip art" and turned up TONS of sites with TONS of free images. One advertises 25,000 images. The first one I tried said it has 7600+ images. I don't know what you want, but you can also try "free educational clip art" or "free holiday clip art" or whatever specific category.
Some of the sites I ended up on were weird... I found 1 so far that isn't too horrible. www.classroomclipart.com
I like those and I aslo use http://etc.usf.edu/clipart/ http://www.abcteach.com/ (I got a deal on a membership) http://www.christart.com/clipart/ http://openclipart.org/homepage
