You can get a MIDI download right here at ff shrine, ill give you the link at the bottom, i think the song your talkin about is Silence before the Storm, it on the disk 2 section.
you can also buy it at the Luca Theater once you get your airship, and you can buy all the movies there too.
also, another cool song is Otherworld, the song played at the opening of the game at the blitzball game.
http://www.ffshrine.org/ffx/ffx_midi.php