There are also a few listed on www.letterboxing.org
Just go to search function and type in "postal."
I received the "Bring 'Em Home" postal box from www.letterboxing.org
a week or so ago, and it didn't have any restrictions on joining it.
You could e-mail the person in charge of it and ask if you can join
the list of addresses for that box. It's a pretty impressive theme
postal letterbox.

moderndulcinea