If you want to proctor for the real MCAT, you would probably need to contact Thomson Prometric since these are the adminstrators of the test. I also believe that this is the company that hires the invigilators and proctors.
If you want to proctor for practice exams, contact the respective test prep company. Although now, I highly doubt many will be hiring proctors, since most of the big companies (TPR and Kaplan) have take at home full lenght exams due to the CBT format.