I don't remember where I found gas....but that's because there was plenty of it, so I never had to worry. Throughout the southern tier there are a few useful rules: first, always fill up because you never know. Second, ask people coming the other way because you never know. Every so often gas deliveries are disrupted, leaving gasolineras closed until deliveries resume.
When you get back onto the eastern slopes of the Andes, you'll notice another rule coming into play: pay attention to the winds, which will cut your gas mileage worse than overloading, gravel, or anything else.
Hope that's helpful.