From Where could I Download ArcGIS Silverlight SDK 3.2?
SDK Downloads | ArcGIS for Developers
-> Log-in with your Esri account
-> Scroll down to the "ArcGIS API for Silverlight"
-> Click on the down arrow
-> Select "3.2"
That being said, I would highly recommend moving away from Silverlight ASAP. The technology is deprecated; it is no longer supported by Chrome, and other browsers will probably follow suit. Esri is dropping support June of 2016.
You should really look at the JSAPI - ArcGIS API for JavaScript