I just wanted to add that you can also get one at Guest Relations at the International Gateway entrance to Epcot, but I have had nothing but bad experiences there. We were there on 8/30 and SIL tried to get one for my nephew who has a medical condition, and the guy in the booth there went on a rant about people scamming Disney on the GAC and he refused to give her one "unless he is in a wheel chair" (he isn't). She offered to show him the doctor's note and he refused, saying "we're not allowed to look at it anyway." She walked away. As we were going through the gate, and before we could go to the Guest Relations at the entrance to the park, get the GAC, and report him, he came out of the booth with the necessary GAC.
Just FYI, that's the only place that we have ever found the Guest Relations CMs to be less than helpful repeatedly. We never experienced someone who accused my SIL of being less than honest about my nephew's medical conditions like this guy, but they have been a little difficult. I don't know why this particular location is like this.
As an aside, my brother accidentally left his GAC in his pocket and it dissolved in the washing machine. He went to GR at City Hall in the MK, and they could not have been nicer to him.