Disneyana.com is not an official Disney web site, the print is available at the Disneyana shop on Main Street, USA. However, you can order it directly from Disney by calling Disneyland DelivEars at 1-800-362-4533.
Oh! That's great! Do you know how much it costs by calling that number? Is there something specific I need to ask for, or if I just say "The Finding Nemo Submarine Voyage Attraction Poster" will they know what I'm talking about?