I heard a CM last month telling someone that sometimes they do run out of wristbands. As the previous poster told you, wristbands are distributed starting two hours before the evening EMH starts. I believe allearsnet.com has the locations. The one closest to the park entrance will always be the most crowded.
Just one more thing about the evening EMH.....They have been a complete nightmare this summer. The wait times are crazier than during the day. Last month we tried EMH at the Studios and left after riding TOT with FP. They reopen the Fast Pass machines for the evening EMH, but after you get one the next return time will be MUCH later into the night. We had far more success getting to all the parks at opening (morning EMH or not). Got a lot done in a couple of hours before the crowds arrived.
Good Luck and have a great trip.