Getting a Marionette-enabled Firefox Build
Marionette is available in all Firefox builds; it is not enabled, however, unless you launch Firefox with the -marionette command-line argument.
Marionette is also available on Firefox for Android (Fennec): the Marionette Python client can launch Fennec for you with Marionette enabled.
Getting a Marionette-enabled FirefoxOS Device or Emulator Build
If you have Mozilla LDAP access, you can download any build that ends with '-eng' from here, or here, or here (depending on build type).
If you do not have LDAP access, you will need to build your own image of FirefoxOS. Follow the Building Firefox OS instructions. You need to be aware that Marionette is not enabled in shipping (VARIANT=user) builds. It is available in engineering (VARIANT=eng) builds, which are the default buildtype. You also need to set this in your userconfig file:
ENABLE_MARIONETTE=1
Getting a Marionette-enabled FirefoxOS Desktop Build
See Using the B2G desktop client. If you elect to create your own build, the only change you'll need to make is to add this line to your mozconfig:
ENABLE_MARIONETTE=1