 snail mail address for the missionaries that your congregation supports, or even missionaries that you might know, and just write them wherever they are serving and encourage them. Since they cannot be at home and do what they do, perhaps the next best thing is to hear from a friend how much they are admired, prayed for, and appreciated by sending a letter or email. I'm sure many of you do that, so I encourage you to keep on doing it for their sake and for the sake of the gospel.