Audio file(?) that separates instruments into tracks

Weekend hobby: I'm making a visualization and audio-control app, where the user can control instruments (or instrument sections) in an orchestra - volume of strings, horns, etc - similar to how an orchestra conductor would do it...

I am looking for sample audio files to use in this app.

1. Is there a particular file format that separates each instrument in an orchestral piece?


2. Is there a repository of select sample files available online?


3. Is there a way to convert an mp3 or other files into multitrack?

I am not sure if the questions I'm asking are in the right track (no pun intended). Should I be looking for files that have multiple tracks per instrument, or just compilations of multiple separate recording files - or something else?