Did you try looking for the Sheet music, or if there's any youtube videos of people playing the song on piano/synthesia?
When I can't find a midi of the song I want, I try searching for sheet music of it as an alternative.
If that fails, all I can do is try composing it by ear, which takes loooong for me. >_<
I still need to learn more music theory. I only know what I've learned from a single piano class, though it does help greatly. XD