If you are looking for general information about MI the best source is either by reading or the website of the international MINT organization, a non-profit network of trainers .
If you are looking for a trainer in MI you have a tougher challenge. Miller and I decided never to try to control the professional behaviour of others, let alone MI itself. Our logic was this: if something is worthwhile, it will last, if not, so be it. While this has enabled diverse application it has a downside: there is no quality control of either training or practice. Membership of MINT does not guarantee the quality of a trainer.
MINT is run by volunteers, and they have produced an excellent guide here: http://www.motivationalinterviewing.org/training-expectations
Motivational interviewing in brief consultations, BMJ Learning
Free 1 hour online module for teaching Motivational Interviewing in brief consultations for GPs, hospital doctors, practice nurses, and other healthcare professionals.
Motivational Interviewing Network of Trainers (MINT Inc.), USA
MINT is an international organisation of trainers in motivational interviewing. The trainers come from diverse backgrounds and apply MI in a variety of settings. Their central interest is to improve the quality and effectiveness of counselling and consultations with clients about behaviour change concepts. Started in 1997 by a small group of trainers trained by William R. Miller and Stephen Rollnick, the organisation has since grown to represent 33 countries and more than 20 different languages.
MI 1% Fund
The idea here is to donate a small percent of your income directly into efforts to help one of the most vulnerable and unjustly treated group of people on earth: children who are dying of HIV/AIDS and TB.