There are 1000's of midi files available on the internet.
I believe that Band-in-a-box will convert mp3 to midi....
Maybe someone that is more familiar with Band-in-a-box
will comment.

From the PGMusic web site:
In Band-in-a-Box 2006 and higher, you can use the
Audio | Import audio... menu item to do import a variety
of different audio file types, including MP3, WAV,
WMA, and CD Audio.

I believe that once you import something into B-i-a-b
you can then export it as midi.  I'm not a B-i-a-b user;
so I don't know who well this works.

You're best bet for popular songs is an internet search
for "midi files"