Take a look at the Open Clip Art Library, the Creative Commons search engine, and the lists of lists that Smashing Magazine is well known for.
http://www.openclipart.org/
http://search.creativecommons.org/
http://www.smashingmagazine.com/category/graphics/
By the way, you haven't told us what type of website you've got. That's important when, for example, you find clipart that has a Creative Commons Non-Commercial license.
