If you are staying in the
Disneyland Hotel (from your other post) you can get them in with your room package. This often works out the cheapest way and is the 'standard' way to do it. The other way is to book room only (often not much cheaper) and buy Annual Passes which probably wouldn't be an option for one day.
ETA - Carrie aren't the Francillien APs where you can't use immediately after purchase? I'm sure you know what you are talking about though - much more knowledgeable than moi!