Or at least using the propellants from shallower gravity wells. Indeed, asteroids are the best places. After that come smaller moons. The Moon, Mars, Earth are

Or at least using the propellants from shallower gravity wells. Indeed, asteroids are the best places. After that come smaller moons. The Moon, Mars, Earth are very deep and steep gravity wells and not practical. The Moon doesn't have proper elements for fuels anyway.