You don't need to Google, since the file is already installed on your computer if you are able to develop Dark GDK applications. The appropriate DLLs are located somewhere in the "redistributable" directory of the SDK.
However, read the DirectX license first to make sure what you can and cannot distribute. IanM has said in another thread that distributing just one DLL, without the whole runtime package, is illegal:
http://forum.thegamecreators.com/?m=forum_view&t=155671&b=22
Also, on some machines you may need to install the Visual Studio runtime files as well. If the release does not start on the other machine and, instead of missing the DLL, it says things like "application error", "invalid Windows application" or "manifest error", installing the Visual Studio files can help. You can find these packages on the MSDN site, but search for the correct version (VS 2005 or 2008, with or without service pack).