I spent 30 seconds on the akc site and found:
You can download a Request For A Duplicate Registration Certificate from our website. You will need to go to the following address - http://www.akc.org/pdfs/masterform.cfm .
The request form is in .pdf format and requires the use of Adobe's Acrobat Reader to view the application. Adobe's Acrobat Reader can be obtained free from www.adobe.com. You must use Adobe Acrobat Reader version 4.0 or newer to view the files.
If you are unable to download these forms, please send your name and mailing address to orderdesk@akc.org and we will mail you a form.
Once you have obtained and completed the form, mail it to the address indicated on it. After we receive your information, we will print a new certificate and mail it to you.
The form will request information about your dog, including its registered name and registration number. If you do not have either piece of information, please contact the AKC at 919-233-9767 so a customer service representative can assist you with looking up that information. We can only release information about a registered dog to its recorded owner. If you are not the recorded owner, you cannot obtain information regarding the dog, nor can you request a duplicate certificate.
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If this dog was purchased from a responsible breeder, then the dog's registered name should include the kennel name. For example, if the dog's name is Dreamwork's Blue Bayou, the kennel is Dreamwork. Once you have this name, try searching the 'net for it.
Finally, if the breeder is a legit breeder, you should be able to track him down through other breeders in the area. Try http://www.thelabradorclub.com
GL!