There is "Midiscan" (actual version is 2.5) from

  Musitek
  410 Bryant Circle, Ste. K.
  Ojai, CA 93023
  Tel. 805-646-8051, Fax 805-646-8099
  price is about $ 300 (?)

A demo version is available (you have to ask/search for it).

Midiscan is an OCR-program (optical character recognition) for sheet
music, but do not expect too much: you need a substantial portion of
correcting work to get good results (with a good experience sometimes you
are even faster putting the music into a sequencer program note-by-note
with a mouse), and it cannot deal with handwritten sheet music.

In my opinion the best way of converting sheet music to Midi would be
playing and recording it with a Midi-instrument.

Claus Kucher