I just want to give a little word of caution here. . .. .I think we all know that this is not a "pen-pal website". Yes, many of us do correspond with some of the great men and women in uniform, but the website advises the following in the Frequently Asked Questions (FAQ) section when asked if supporters should expect a thank-you when they send something:
The Soldiers are under no
obligation to reply. They can't be. We can't burden Soldiers who don't
get enough sleep, don't eat enough, live out in the open, or whatever,
to do anything not related to their job. Sure can't have their
leadership tell them to reply either. If you get a reply, it is
voluntary by that Soldier.
Also, AnySoldier is not a "hook-up" or "love-connection" website. I just say that, because someone who requested an address previously obviously thought that because 1SG John P. Emswiler wrote the following post:
One warning...I am not looking for a
"penpal" or someone to "hook up" with. I have received letters from a
couple of women who are looking to "make a connection" with a soldier.
To them I must say that I am married to the most wonderful, beautiful
woman in the world and that I am not interested in seeing what is out
there so, thanks for your support, but please don't write for those
reasons.
I think if you're looking for a pen-pal, then I would first look for those groups who are requesting pen-pal's. Go to the "Where to Send" page. Click on the Search button at the top. In the box that shows "requests and email contents" type in pen-pal. This will bring-up a few requests. Then go back and type it in with the "-" like this penpal, and you will find more. This way, you know that you're writing to a group who are looking for pen-pal's. If you write to someone who does not request pen-pals, then certainly send them a supportive letter, and you might mention that if anyone is interested in corresponding that you'd be happy to write, but again, don't have high expectations, because they didn't ask for that.
If you are a person who likes to write, then that's GREAT! I think you might want to write to several different groups, who are requesting pen-pal's, so that your chances are greater that someone will actually write back. Who knows. . . you might find a few people who enjoy writing! 
George
P.S. Postage to service members with an APO or FPO address is the same as U.S. postage, no matter where they are in the world. . . . .so it's .42 cents for up to one ounce. If your post office is like mine, they don't have a very good selection of stamps, so you can find the most current and new-issue stamps at www.usps.com Click on the tab at the top Buy Stamps & Shop. They have lots of other fun items too.