To the OP- I worked as a proctor for EK and found the job through craigslist. You can probably also find on each test prep website about opportunities for jobs (mostly tutoring/teaching ones) but Im sure there might be links for proctoring or at least an email to contact.
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.