Where can I find source code for .net CLR and C#?

You can download the source code of .NET with the help of Netmassdownloader

You can use Reflector to view them.

Or, if you have Resharper 5 installed, you can download the source code for viewing purpose.

_{(source: jetbrains.com)}

But no, it is not open source, despite that you can download and view the source code.

Part of .NET Core is now open-source, starting with .NET Core 5.0.
Here is the project site: http://www.dotnetfoundation.org/netcore5
Here is the code: https://github.com/dotnet/corefx