We tend to ride in out-of-the-way places when exploring the mountains of Mexico. Most of the little villages we ride through don’t have an actual gas station.  But that doesn’t mean they don’t have gas for sale.  Every little pueblo has somebody selling gasoline from a 55 gallon drum.  Ask around and one of the locals will point the way. Once you get there, just tell the man how many liters you want. I recommend straining the gas with some cloth or a t-shirt.