How about the ones that come with OS X? The ones that are in the Beach screensaver.
Go to your hard disk> System > Library > Screen Savers
Right-click on Beach.slideSaver and choose Show Package Contents. Go to Contents > Resources and all the images are there.
If you're looking to use one as a desktop, just go to /Originals/ the full size images are located there. (thumbnails open up to 640x480), So you can look through and any you like, the navigate to /Originals to get the fullsize.