I've been looking all over the place and I can't find the download!
Where do I find the ArcGIS 10.1 SP1 for Server Web Adaptor?
You need to Login to the ESRI customer care portal and then navigate to Products & then download the ArcGIS Server Web Adapter.
You need to download first Web Adaptor and then apply Service pack 1
Log into your customer care portal navigate to products tab, then click on downloads. Search for the dsired version of ArcGIS for Server Enterprise (Windows). You will then be directed to a page with options to download the individual components of ArcGIS for Server. The Web Adapter for IIS will be listed among these downloads.