Go to www.allmoviephoto.com/photo/2003_Finding_Nemo_photo.html and scroll down to the bottom of the website. You will find several movie stills to click on there.
Purchase the "Finding Nemo" Read Aloud board book by RH Disney. This book has several movie stills from the movie. While geared towards very young children, the stills are well done.
Go to www.123-free-download.com/download/screensavers-tools/finding-nemo-movie-screensaver/4152.html and download this freeware. It has 16 still shots from the "Finding Nemo" movie.
Go to a poster specialty story, such as Barewalls or Ye Olde Poster Shop, and purchase "Finding Nemo" posters. This is a great way to get still shots from the movie. Since they are poster size, their larger-than-life size can be very fun for kids.