In order to get the source for officially released versions, go to the Files tab in Rugged forge and select one of the rugged-{module}-x.y-src.zip files. The module part is the module (you need at least the core module, but other modules may be interesting for you too). The x.y part in the name specifies the version.
The development of the Rugged project is done using the Git source code control system. Rugged Git master repository is available online.
You can browse it using the Repository tab in Rugged forge
You can clone it anonymously with the command:
git clone https://www.orekit.org/git/rugged-main.git
If you are a committer, you can clone it using your ssh credentials with the command:
git clone ssh://git@www.orekit.org/rugged-main.git