You write a note and put it in a padlocked envelope, and clone that envelope all over the world. Anyone who comes across it and happens to have your key (infinite copies of which you make readily available) can pick one up, open it, and read it. 

It's your stationary, your ink, your padlocks, your key. Nobody can take those away from you. All they can do is try to find places you left envelopes and burn them.