Where to download ArcGIS Web Adaptor?

You need to Login to the ESRI customer care portal and then navigate to Products & then download the ArcGIS Server Web Adapter.

To be specific, please login to the portal, then go to My ESRI page (default landing page), then go to My Organizations -> Downloads. From there, select the required version of ArcGIS server based on the platform (Windows/Linux), and click View Downloads against that. For instance, click the link against "ArcGIS for Server Enterprise (Windows)", then you will be taken to the page where all downloads for it are listed. Now select "ArcGIS Web Adaptor (IIS)" or "ArcGIS Web Adaptor (Java Platform)" based on your need. That's it.

Please check Web help and the installation video

You need to download first Web Adaptor and then apply Service pack 1

thanks..