ESRI released the Toolkit for the ArcGIS Silverlight at codeplex for all users to be able to download the source code.
Go and download the source code here.
This project contains the source code and libraries for the ESRI ArcGIS Silverlight Toolkit SDK. The library is an extension of the ArcGIS API for Microsoft Silverlight™/WPF™. Included are a number of controls, widgets and tools that you can use to enhance your web applications.
NOTE: This library is only an extension of the ArcGIS API for Microsoft Silverlight™/WPF™, so in order to build a complete web mapping application, you will also need the
ArcGIS API for Microsoft Silverlight™/WPF™. See below for download instructions.