The short answer: Autodesk no longer sells or distributes version 9.1. However, there are two legal alternatives:
Once the download is complete, extract the zip file using a file archiver like WinRAR or 7-Zip: Download Artcam 9-1 Pro Zip File
The safest place to start is the official Artcam website or the developer's site. Look for a "Downloads" or "Products" section where software versions are listed. The short answer: Autodesk no longer sells or