Get your GAC's at the parks. There is a listing posted on the top of the disabilities forum in the FAQ's Sticky with lots more info you will find helpful.
As was already mentioned, Guest Assistance Cards (GACs) are only available at Guest Relations at the theme parks (not water parks).
And, you will find more information about GACs in post #6 of the disABILITIES FAQs thread near the top of this board.