I saved some Christmas cards with nice artwork. I followed the link on the back of one to a site called
http://www.image-source.co.uk/. They have some really nice artwork - I had to sign up though so I could browse around.
I'm not an illustrator, so I won't be contacting them - but if you are interested in learning more, there is an area on their site with info about becoming a contributor.
You can view (and right click) a very large image though and there is no watermark.