Go to the nightly build download site:

Click the link for the build you want.

You will be shown an index page where you can find a link to the folder with ZIP archives of binary builds and sources.

Click the link or add "zip/" to the end of the URL in your browser’s address bar manually and hit enter. In other words, the complete URL might look like this:

There are about a dozen links on that page. The one you want begins with "netbeans-trunk-nightly" and ends with "-src.zip" Click that link to download the source archive.