If you don't have MapSource software installed you can download an update package from the Garmin website. This update package can't be installed as a fresh copy by simply running it.
Happily, there is a way to use the update package they offer on their website to install a fresh copy. Just follow the steps below and you should be able to install a fresh copy.
Download the package from https://www8.garmin.com/support/download_details.jsp?id=209
Do not run the installer after the download is finished
Find the downloaded package on your computer in File Explorer in Windows.
Open the MapSource_6163.exe with archive software such as 7-Zip, WinRAR or PowerArchiver. You can right click on the file and extract it.
Open the extracted MapSource_6163 folder and first run the MSmain.msi installer. Wait for it to complete and then run the Setup.exe. Follow the instructions to complete the installation.