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.