So you have to pay an extra 5% on the fare (you can exceed MPM by up to 25% in 5% increments, with a corresponding fare increase).
All of the MPM values are published, but the price is a bit steep (USD 2,400 for the book and USD 50,000 for the database). See
http://www.iata.org/ps/publications/mpmced.htm
Guess that is the tool you'd want, but not quite at that price. Given the cost it is understandable that you don't find any web sites offering access.