Launch contracts. They sell a service (sending satellites into orbit) for more than it costs them, and then reinvest the profits into crazier things like reusable rockets and getting to Mars.

They launch things into orbit in exchange for money.

Investors, stock, government contracts, and offering launch contract services to private companies. I would also not be surprised if they have private contracts with companies and governments for research and development purposes (visualization, simulation, automated testing with their rockets for instance).

I remember them getting a NASA contract recently. Here: http://spacenews.com/spacex-wins-contract-to-launch-nasa-ear...

They probably make a bunch from licensing their various proven technologies for other uses.