Hi,
You can get source code by executing:
$ echo "deb https://repo.download.nvidia.com/jetson/ffmpeg main main" | sudo tee -a /etc/apt/sources.list
$ echo "deb-src https://repo.download.nvidia.com/jetson/ffmpeg main main" | sudo tee -a /etc/apt/sources.list
$ sudo apt update
$ apt source ffmpeg
Please give it a try.