...(snip)...
There is a considerable time lag between ordering the DDE card and actually receiving the card, but they send you a fax or email confirmation which you can use the same as the card. I believe the card expires about 13 months from the time the card is physically manufactured, so you could effectively get 14 months or more out of the card if all the planets are in perfect alignment.
I think it is possible to wait until you are actually on-site to buy your DDE, but I'm not sure about that. Hopefully someone who has actually done it can answer that for you.
Good luck and have a great trip.