The sooner in advance that you purchase your tickets the cheaper they be plus it will reserve your seats. They all go by reservations now.
You can purchase the day of but you have to be sure there's seats available. I recommend purchasing at least a day or two prior to leaving. You can purchase online too.
You can purchase up to a half hour before departure but if you don't buy sooner, they might not have a seat for you. People travelling with wheelchair or service dog must buy 24 hrs in advance.
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