If you can try writing him, there are a few addresses available which have been tried out. Sadly, ALL of them just auto signed pics - Johnny stopped signing by himself some years before.

• Always enclose a Self Addressed, Stamped Envelope (SASE): The SASE can be used as a return envelope for Johnny's reply (if any). If you send along your own item for his to sign, make sure the SASE (with the correct postage amount) is large enough for the item.
• If you don't send his an item to sign, he might send you a pre-printed photograph. What's the difference between a pre-printed, signed photo and a signed photo? The pre-printed photo is a mass-duplicated photo which has been copied from a signed photo (but the signature is still authentic, of course). If you send his your own item to sign however, the signature will be unique.
• Keep in mind that if you order a signed photo from an autograph dealer, you might risk getting a photo with a faked autograph (despite authenticity 'guarantees'). If you're really interested in a genuine autograph, it's worth spending some time and money by sending a letter to Johnny himself (which may be even cheaper than buying through dealers). At the same time you'll let him know that you appreciate him.
• It may take several months before you get a reply, if any.

Source: FanMail.biz | JD Collective