All of the terrain is scratch built. The islands are made from sheets of polystyrene.
The small islands are plastercast rocks made using tin foil as the molds. The temple objective is a plastic fast food container with a polystyrene dome an then covered in plaster with the detail scribed in.
The trees are mostly taken from aquarium plants often broken up to make more useful shapes.
It's quite possible to make terrain without needing to go to a shop and buy it.
Most of the scatter terrain comes from the mantic terrain crates. That terrain is good value for money.