In the long term SpaceX plans to generate it using the sabatier process by pulling CO2 out of the air. It's old physics, and the tech to do it on a large scale

In the long term SpaceX plans to generate it using the sabatier process by pulling CO2 out of the air. It's old physics, and the tech to do it on a large scale will be required for their ultimate goal of going to Mars (errr... well, it will be required for a Starship to be able to come back to Earth from Mars)