Or those who simply want it, Ichinosekai.net has compiled its very own
Amazon.co.jp A-store featuring all of the latest Piano no Mori merchandise! By which I mean, collected all of the Piano no Mori stuff on Amazon into one place so it's easier to find. This includes the Japanese tankoubons, the movie DVD, and various music CD compilations. Be warned though - Japan is a different region from America, so unless you have a Japanese DVD player (or a Chinese one, which is lazy and ignores region codes entirely), you might want to think twice about the DVDs.
Still working on compiling something similar for the Hong Kong and Taiwan versions. As Piano no Mori is not serialized in English yet, nothing equivalent will be available. For now, go read KEFI! Hopefully if this series becomes popular enough, some American company will notice it and take over the job. (I hope it's Dark Horse.)