Here are some resources that you can use to send Christmas cards to soldiers:
-The American Red Cross has a program called "Holiday Mail for Heroes" that collects and distributes cards and messages to service members and their families. You can learn more about this program and find out how to send cards here: https://www.redcross.org/take-action/donate/virtual-red-cross/offer-support/offer-support-to-service-members-and-veterans/holiday-mail-for-heroes.html
-There are also several organizations that specialize in sending care packages to soldiers, and they may also accept Christmas cards. Some of these organizations include Operation Christmas Child