All code in Ubuntu and Mythbuntu is distributed in Debian binary and source packages. This means that the code is taken from an upstream (such as MythTV) and some modifications are layered on top of it to make it integrate and build on a Debian or Ubuntu build system such as the Ubuntu archive. Rather than duplicate already existing information, it's highly recommended that you review the following resources:
All Mythbuntu packages are intended to be compliant to specifications detailed above. They can be obtained using standard commands on any Ubuntu system.

Information about where individual packages are stored can be found on the Developers Cheatsheet.