Oh! I have a suggestion! ^^
http://www.youtube.com/watch?v=mrESIdV8ZuY <-- One of the most beautiful song pieces I've ever heard on the piano. It's from the anime Clannad and Clannad: After Story. And that anime does have alot of lovely piano songs in it, too. So it's not just this one! I recommend you listening to the anime' soundtrack. It's beautiful.
But anyway, I play the piano myself and have been trying so hard to master this one down. I have already learnt how to play another song from the anime on the piano and I want to try this one. I'm just suggesting this one for you because..well, I just think I would like to share the beauty with you!
I usually go on YouTube and watch tutorials on there, so I can't recommand any good websites where you can find the sheet music. But there's plenty anime fans out there that probably have posted it somewhere on the internet. Have fun!