Not only can you send cards to members of the military stationed overseas, you can make them throughout the year. There are plenty of places to look online, especially scrapbooker and cardmaker blogs. Local scrapbooking and craft stores also organize card-making events. There are even specific sites dedicated to this idea. The idea is to make handmade blank cards in batches so soldiers can send greetings to their families.
Here's an official site to check out: http://www.angelstampersforsoldiers.com/as4s/