Where to find the source code for .NET Framework?

If you are looking for the source code of .NET Framework (which has been open-source for a while now), you should take a look at the web page called "Reference Source". This is available at https://referencesource.microsoft.com.

This web page contains a browser that allows you to navigate the source code, but you can also download a single ZIP file for all the released .NET Framework versions since .NET 4.5.1. The packages are about 50–60 MB in size.

Hope this helps!