... rain and from evaporation of standing water. Of course you'll need an atmosphere to encourage the recycling process... rain.
Over time, rain helps to brake down rocks, disolves them. In the process, sodium is produced and when evaporation occurs, you have salt. It is also found undergound in caves in the form of rocks after water from rivers percolate down and long enough for the rock salt to form.
Salt requires water, steady amounts of water over time.
Why didn't something like that happen on the Moon? Impact events may have briefly created liquid water on the moon, but I highly doubt that the moon had actual periods of rain long enough to produce salts and I highly doubt that water could be on the lunar surface long enough for salts to occur in ponds. The moon would have needed a dynamo to support an atmosphere.
I don't think anyone knows the full stories of Ceres and Vesta. Although Ceres is spherical, it is very small. Was it captured late by the asteroid belt after it formed? Same for Vesta?