I stockphoto has great illustration images. if you cant afford to buy one, some of them are big enough in previews and easy to edit to take watermark out.
You can also contact artists directly. For example, browsing art websites such as Deviantart. If you find something nice, just send off an e-mail to the artist letting them know that you'd like to use it for whatever purpose. Most of the time people are flattered to have a request for their work and offer it for free (just make sure you explain in detail the purpose of the image, and provide a URL to where you wish to include it). I've only done this once but was quite happy with the results, I provided a link back to the artist in exchange for a very nice piece of art.
Depends on what kind of website you're working on. Is it a community site, government site, gift site like mines, commercial ones, personal ones like myspace...depends.