The SessionDataSource is a custom implementation we have for the Live demos app. It is actually just a wrapper of the asp:SqlDataSource control, and you can easily convert to the SqlDataSource by :
Replacing the asp:SessionDataSource with asp:SqlDataSource
Removing the DisplayWarning="false" and PrimaryKeyFields="ID" properties
If you need the SessionDataSource file, you have to install the Telerik demo suite on your local machine. You have to log in with your credentials to the Telerik website and enter the "Product Download" section, download and install Telerik_Demo_Package. Once installing the demos application, you can locate the file at (Live Demos Directory)\App_Code\QSFCommon\SessionDataSource.cs.
For your convenience, I have also attached the SessionDataSource.cs file to my reply.