How do you find a physical address when you only have a PO box address?

Surprisingly easy: http://www.blackbookonline.info/bbo_poaddress.aspx

This link only provides the address for where the PO Box is located, it does not provide the physical address of the PO Box holder.

Only attorneys and collection agencies can really obtain that information. If you're collecting a debt from an individual, a collections attorney can request information from the post office for the physical address associated with the PO Box. But you, as an individual, cannot access that information.