The tickets as far as I know are only available on a first come fist served basis in the wee hours of the morning you want to attend. Appartently there are only 15 seats available for each showing. I'm not sure about the location of the Guest Services window (where you have to buy the tickets) It was a little white building next to the tickets booths before, but now that the ticket windows have been remodeled I'm not sure any longer. Maybe someone can give you further info on that.
My DH had to go one summer at 4 am!! You might want to call and ask where the Guest Window is, the last time its was on the outside to the right of the gates when you walk in Disneyland, I am not sure if its been moved.
It was worth it!! I can never see Fantasmic any other way, sipping my mint julip while decieding what dessert to have.. It was just wonderful!!!!