Get the DLLs from the Global Assembly Cache (GAC)

You might have some software installed on your server, and registers the assemblies to the Global Assembly Cache. Typically, installers would register the assemblies but remove any accessible copy from the local disk as everything is packaged inside the msi or setup file.

There is a really simple way to extract the dlls from GAC, though.
Launch MS DOS, and change directory to c:windows\assembly\gac_msil (you won't be able to open this in windows explorer)!
run the xcopy command. Example:

Note: there are alternative ways such as modifying the registry, or temporarily uninstalling certain dlls to allow browsing from windows explorer, but I would seriously advise against that.