In 2013, CAMduct was also shipped on DVD. If you or your shop has the original installation disc, that remains the safest and most legitimate source.
If you absolutely must use 2013 to support old machinery or job files: Autodesk Fabrication Camduct 2013 Download
It is worth noting that in 2013, this software was often referred to as or CAMduct under the Autodesk brand (after they acquired Map Software). Today, it is part of the Autodesk Fabrication CADmep or Fabrication CAMduct product line. The file formats and database structures have evolved significantly since 2013, meaning: In 2013, CAMduct was also shipped on DVD