you can find Microsoft.SharePoint.IdentityModel.dll in the GAC on SharePoint 2013. Note that the GAC location has changed since .net framework 4.0. It's now in c:\windows\microsoft.net\assembly
Here is the location:
C:\Windows\Microsoft.NET\assembly\GAC_MSIL\Microsoft.SharePoint.IdentityModel\v4.0_15.0.0.0__71e9bce111e9429c
