Use Google and search for Adobe Lightroom Update downloads. Then go to the Adobe download site and download the most current version of LR version for the serial number you have.
Adobe has made this very hard. They always point you to the Creative Cloud version and you need the Stand alone version.
Here's the link.
Is there any chance that the version of Lightroom you are trying to install was purchased as an upgrade? If it was, then you will need your original Lightroom serial number as well as the serial number for the upgrade version. And you will need to download an upgrade installer. But I don't have a link for that.
THANK YOU! I knew this had to do with Creative Cloud and you were right. I followed your link and all is well. I very much appreciate your "Bondness