It's great to see you here today at planDisney, Natalie! You must be so excited about your upcoming trip to Florida. I'm sure it has been a long time in the making!
Natalie, you're right, one-day tickets are not available for purchase outside of the United States at this time. Since you're only planning on being at
Walt Disney World Resort for one day, I recommend purchasing your tickets as soon as possible when you arrive in the country. This is my recommendation because, on top of having a valid ticket, you will also need to have a
Disney Park Pass reservation for the Theme Park you want to visit, and the earlier you make it the more availability you will have to choose from. You can still arrive at a Theme Park on the day you want to visit and purchase a ticket there, but it is possible that they may be out of Disney Park Pass reservations for the day which may leave you having to make some quick decisions early in the morning. If you purchase your tickets in advance, even just the night before you plan to visit, you will know that you have all of the requirements ticked off in order to visit the Theme Park of your choice on your preferred day.
If you purchase your tickets in advance, you will also have a valid ticket in your My Disney Experience account should you want to use
Disney Genie+ service on the day you're visiting. Disney Genie+ selections can be made beginning at 7:00 am on the day you're planning to visit, so it would be quite advantageous to have your tickets purchased and linked to your account by that time if you're planning to use it.
I hope this answers your question, Natalie, and please let us know if there's anything else we can do to help you plan your magical vacation!
Jessica