Where can I find Microsoft.TeamFoundation.Build.Client in Visual Studio 2015?

Microsoft.TeamFoundation.Build.Client.dll still ships with VS 2015. For the 2015 release, we have removed the client OM DLLs from the GAC.

In the RC release you can find the DLL in c:\program files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\CommonExtensions\Microsoft\TeamFoundation\Team Explorer. It's kind of an obscure place.

The TFS 2015 client OM RTM NuGet package is now available (also works for Visual Studio Online):

.NET client libraries for Visual Studio Online (and TFS)

I do show a Microsoft.TeamFoundation.Build.Client assembly version 12.0.0.0 that can be referenced. You may need the assembly from Visual Studio 2013.

I have updated all my build processes from TFS and VS 2013 to 2015 and they run, yet I may not have custom tasks, etc. as you do.

Visual Studio 2015 does say it supports all previous build process features (they now refer to as XAML Builds), even though they have re-written the build process. See the TFS 2015 release notes for details: https://www.visualstudio.com/news/tfs2015-vs#vNextBuild