"My 22-year-old son has been unable to complete college courses or vocational training. How do you find a coach and determine if they are good?"
Check out adhdcoaches.org and ADDConsults.com for coaches in your area. Much coaching is done by phone, so unless seeing someone in person is absolutely a must for you, finding one you are comfortable with should be doable.
Also, "Guiding Principles for ADHD Coaching" is now posted on the ADDA website and it is the FIRST endorsed document from professionals in the field that guides you as far as what coaching is and how to know if you are choosing a coach that will meet not only your needs, but abide by certain standards.