This is based on what I have read here, not on persoanl experience. First, I believe you can buy a ticket at the same window where you turn in your vouchers. That would be the Will Call window at MK, if I am not mistaken.
Also, are you sure you want to go to MK on an EMH day? It is my impression that those days tend to be more crowded. Most posts I have read onthe subject advise going the day after EMH.
Hope you have a great trip!