For starters, this is worth repeating....the most comprehensive image search tool on the web is:
http://images.google.com
But for ordering prints of artworks, I suggest that you check out the following:
AllPosters.com
Printmaster Inc.
Barewalls.com
AllAboutArt.com
Art.com
PosterGallery.com
ArtPrintCollection.com
Web Gallery of Art
Art Renewal Center
Meanwhile, many Museums carry posters of the masterworks in their collections...so be sure to check out museum shops too.