One of the best places I know of with Midi's of Piano Concertos is located at: http://kunstderfuge.com/midi.htm
This one is the best one I know of because many of the contributors have put a great deal of effort into interpretting the pieces and making them sound musical (i.e. not just entering in the notes).
Another location I know of is:
http://www.classicalarchives.com/
in general, the quality of classical midi files on the webcan vary significantly. Many of them sound very mechanical or just wrong. Some sound somewhat realistic and a few sound good. They all fall short of real performers and perhaps they always will. It is astounding to me, however, that I can fit an entire orchestra and piano inside a small laptop computer.
I have used Pianoteq with some of the midi Piano Concertos I have downloaded and I think Pianoteq sounds quite good. It is challenging (perhaps impossible) to balance orchestral sounds and Pianoteq with these Midi files since the setup used by the midi file authors is guaranteed to be different than my setup. On another thread, it was noted that midi files work best with the instruments they were created for. This problem is magnified dramatically for piano concerto midi files. I am very much an amateur and have a lot to learn.
I did a quick search to see if someone had sequenced the 3rd Piano Concerto of Prokofiev, but I did not find it. When I listen to these concertos, I use Cantabile, Pianoteq, and some free VST's/soundfonts that are out there.