They generally give these out within a few hours of the park opening. One of the best ways to get one is to be in a remote area of the park where not too many people pass through.
On two occasions we got these passes while walking on the path that leads between Fantasyland over towards Big Thunder Mountain. There is a Cast Member entrance on that path. Look for employees holding black bags over their shoulder. They just give them out to people passing by.
Another good way to find where they are handing them out is to keep your eyes opening in the morning. If you see people who have them on, ask them where they got them. Often times it can take 15-20 minutes for them to distribute in that location ... giving you time to head over and get some for you.