You can buy domesticated Mallards and lots of ornamental too like Wood ducks but most keep ornamental in a netted aviary because they are expensive.Lots of hatcheries sell Mallards though
Ducks have to be locked up at night in a secure building if not they will be picked off as you have seen. They have no means of escaping a "night predator" or stalker even out in the middle of the pond something can take them either from swimming out or from flying in or from underneath, The...