The latest version of Parallels Desktop for Mac is available for purchase at the Parallels Online Store:
You can choose between a permanent license and a subscription. Please visit KB 122929 to learn more about the difference between these kinds of licenses and decide which one you prefer.
The Parallels Online Store offers an electronic copy of the software. When your order is completed, you will receive a download link and an activation key for Parallels Desktop for Mac.
Note: Parallels Online Store always offers the latest version of Parallels Desktop for Mac. To obtain one of previous versions of the software, please contact your local software resellers to find out if they still offer it.