Veronica,
Technically you can buy theme park tickets at the gate, on the day of your visit. However I would strongly recommend against doing that, if it can be avoided.
Why? Well, Disney now requires that you have a theme park reservation via the
Disney Park Pass system for any day that you want to get into a theme park. You would not be able to enter a park, even after purchasing a ticket, if space was not available.
To show you why that would matter, you need only to look at the
Disney Park Pass Availability Calendar at the time that I am writing this answer. Sure, for the date in August when you are arriving, all four parks have available spaces for you to enter. But that is almost a month and a half from now. If you were to look at the calendar for today, you would only see
EPCOT available. And for the next 5 days after today only EPCOT, and in some cases
Disney's Animal Kingdom Theme Park, have availability. Now those may be what you want, but as you can see, there can be a high likelihood that the park you want to enter that morning may already be full for the day.
My advice is to buy the tickets as soon as you know your plans are cemented down, and then immediately make your park reservation selections. It is also a good idea to check that calendar before purchasing the tickets, to be sure that the park you want to visit has availability.
BONUS TIPIf you meant to ask about tickets for a water park, like
Disney's Typhoon Lagoon, you certainly can just buy tickets that morning. Technically these parks can also reach full capacity, but I don't think that happens very often, and I don't think you have to worry about that.
Write me back if you have any follow-up questions.
Have a Magical Vacation!
Doug