Contact the manufacturer and request a copy of a NAFTA Certificate of Origin or a Producer's Affidavit written on company letterhead stating that the product (include the HS number and description) originates in the United States and qualifies for NAFTA.
Using the NAFTA Certificate of Origin or Producer's Affidavit from the manufacturer then fill out a new Certificate of Origin using the information provided by the manufacturer and keep the manufacturer's Certificate of Origin on file.