Where can you get postal service stickers?

You know like big ones? The size of a small bumper sticker?



And is there a law stating that you can only put them on envelopes? I kinda just wanna use them to stick on different things and be random.Where can you get postal service stickers?
at a POST OFFICE, but they only put the big ones on if you are mailing a big package/box. You can ask them, or they might just have them in the post office section where u can buy boxes and gift wrap. HOPE U GET SOME P.O. Buper Stickers. :)