If you have an extra tank or access to a cheap/free tank just make one. however 2 of my local salt stores sell a pre-made setup by a company here in town. Something to consider...
Sump is imo the best filtration unit for a marine tank. You get to keep all your equipments there.
Skimmers, reactors, heaters, refugium... you name it
Something to consider when you make the sump is how much water will flow into the sump if the pump gets turned off; you wouldn't want the sump to overflow. You also need to consider how much water will flow into the aquarium should the overflow get clogged.