Where can I find free source code online to find out

I started this Pluralsight course about testing a .Net application (desktop application, WinForms) with NUnit because I want to learn how to test code (I am a tester). The project I'm currently working on has a huge application and the source code is too complex for me right now. Where can I find the source code for the desktop application (open source I think) so I can start learning to unit test by linking this code to my test project in Visual Studio. Thank:)