FastECU source code is hosted on github under GPL-3.0 license. Different versions of software is found on projects release page.
Definition files has to be downloaded separately. There is few ways to get definition files: RomRaider and EcuFlash homepages or more updated from github.
RomRaider definition files from RomRaiders homepage can be found here.
EcuFlash definition files comes with EcuFlash install and can be found here.
More updated definition files to both can be found from RomRaider user td-d repo on github.