When you install MariaDB Galera you need to use a repository that also provides the Galera packages. They are in this repo indeed, see http://mirror.aarnet.edu.au/pub/MariaDB/repo/5.5/ubuntu/pool/main/g/galera/
The issues seems to be that there are only binary versions, no source packages for Galera.
This repo does not contain a version of MariaDB nor Galera that I've packaged, but I think you should be able to install simply by running 'apt-get install mariadb-galera-server' and not via source commands. If you take the path of compiling yourself, then the package manager isn't very useful anyway. The source of Galera can be downloaded manually from github. My pull request does not fix anything related to this repo layout or contents.
also: