You'd likely start with the near-Earth asteroids. The easiest have the same delta-v requirement as the Moon. SpaceX is planning about a dozen refueling flights

You'd likely start with the near-Earth asteroids. The easiest have the same delta-v requirement as the Moon. SpaceX is planning about a dozen refueling flights for a Moon landing but that's for a hundred tons of payload. For the main belt, it probably makes the most sense to use low-thrust high-ISP rockets rather than chemical.