You cannot download VSS for free. It is part of the Microsoft Visual Studio products. The only way you can get VSS is, purchase Visual Studio.
The VSS trail version is currently discontinued. But you can download a trial version of Visual Studio TeamSystem from this URL - www.tryvs2005.com/profile.aspx?
If you are looking for any free products, you can find several free source control systems from internet. Both VSTS and VSS are not free.
One of the popular free source control systems is CVS. You can download CVS from the URL - www.nongnu.org/cvs/.