OBM development team uses de git version control system for its sources. You can easily get the source code using it.
The first thing to do is to setup git on your computer. Using Linux, it should be as simple as :
apt-get install git
if you're on Debian/ubuntu flavor, or
yum -y install git
if you're running a RedHat/Fedora distribution.
Use the clone command of git to get the source code of OBM :
git clone http://git.obm.org/obm
You then have a obm folder containing all the sources.
Use the clone command of git to get the source code of the OBM Thunderbird/lightning connector :
git clone http://git.obm.org/lightning-connector
You then have a lightning-connector folder containing the sources.
Once again, use the clone command :
git clone http://git.obm.org/lightning
You then have a lightning folder containing the sources.
git clone http://git.obm.org/roundcube
You then have a roundcube folder containing the sources.