Oh, that's not what you asked. You said "The address of the Post office where the PO box belongs to"
If you want to know the address of who is renting the box, I believe you can go to your local PO, and request that information. There might be a small fee. I'm not sure if they will reveal the information if the owner is a private individual, but I think if the box holder is a business they will tell you.
Your best bet is to ask at your local PO.