When trying to get them from http://maven-glassfish-plugin.java.net/svn/maven-glassfish-plugin, all I get are svn OPTIONS 200 OK error, indicating that SVN server is not where I think it is. So, how can I get the sources to do the modifications I need ?
Here's a tip about the Subversion repository access for projects at Java.net. The SVN URLs (in HTTP/HTTPS) published on the project pages are valid; it's just that they are not browseable.
For instance, the Maven-Glassfish-plugin has it's SVN repository URL listed as http://java.net/projects/maven-glassfish-plugin/sources. Browsing to this location in a browser will get you nothing.
But if you execute svn log http://java.net/projects/maven-glassfish-plugin/sources you will see that you can access the repository; other SVN commands will also work. Hope this helps.