Well I play Keyboard/Piano for a long time and I love to play the themes from anime series. I've already played themes from Air, Naruto and Kimi Ga Nozomu Eien.
But finding good sheet music of Shinkai's work is really hard, well it's actually Tenmon that composes and plays the music so the credits go to him...
I only found two decent sites, but I'm still not satisfied with them (yeah I'm a bit spoiled =P )

So please if someone knows a good way to get a hold of it please tell me (kidnapping Tenmon is NOT an option!)

Here are the links of sites that I've found:
Josh's anime sheet music collection : This site is wonderfull, it has sheet music and .midi's from almost every anime!

L'indestin : This one has some sheets from Voices of a Distant Star, but it are only draft versions

ZOMFG!

lukas8me

ZOMFG!

Oh yeah thanks... you helped me out al lot

Like... anybody?!?!

I'm in a serieus crisis here and not even one usefull reply! Seriously that hurts guys (and girls)...

*Starts working on his wallpaper for the Shinkai Offical Wallpaper Contest in the meantime*