Usually when a person moves she will have her mail forwarded to her new address. But there are times when a person would have their mail forwarded to a Post Office Box address until they get comfortable in their new home. Any mail that is sent to the old address will be returned to the original sender, with a notice of the recipent's new address. The Post Office has no system in place to allow an individual to obtain another person's forwarding address. But there are some indirect ways to obtain this information.
Write the person a letter asking for his new address and send the letter to his old address. The letter should be forwarded to his Post Office box and if he responds you will have his new address.
Wait until the forwarding expires and address a letter to the person's old address. Write "address service requested" under your return address. The Post Office will only forward mail from the old address to the new address for a limited time. Once the time has expired the Post Office will send back your letter with the new forwarding address.
Search free websites such as MelissaData.com or ZabaSearch.com. These websites generally have people's most recent addresses. You will need to know the person's current city and state to use these services.