You can apply for one online or at the post office. You will need to fill out the form and two valid form of Identification. Prices varies base on the size of the box you want.
A caution: don't assume the one they assign you online will be the one you get. We applied for one and started using it for forwarding, but then they changed us to another one when we went in person and we never recovered any mail sent in the interim. They refused to even look for it. So proceed with caution and don't use the PO Box number until you have your hands on the key.
Thanks! Your answer is awaiting moderation.