Where can I find Microsoft.TeamFoundation.VersionControl.Client.dll in Visual Studio 2015 installation?

These have been moved into a Nuget package.

They're also stored under the "CommonExtensions" folder of your Visual Studio installation, but these are not meant to be referenced anymore, that's what the NuGet package is for.

 C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\CommonExtensions\Microsoft\TeamFoundation\Team Explorer