The ocean is a major source of energy which could be used to power the thing.
And I'd prioritize ocean cleanup over landfill, first of all. I mean, maybe we use the ocean to construct the fleet, and then when its nice and clean, send the fleet to land ..
Landfill is also a source of energy. Some cities have built power plants that are fueled by landfill emissions or through incineration of excess materials.
Oh, this is just a thought experiment, the engineering isn't within our grasp.
I'd imagine some sort of floating device which can build copies out of itself with the plastics it finds is not far away, but it probably does involve a fair bit of science before it becomes more than just fiction..
