I personally like the Billboard trees and plants by XfrogPlants though they are expensive at $15.00.Foleypro also has a  forest kit over at DAZ..It's called Foleypro's 2D trees and costs less than 10 dollars.

Otherwise you are stuck with making your own 2D trees as croowe has suggested or isolating individual tree photos to make your own billboards on individual 2D planes.Or you could just put a whole forest photograph onto a 2D plane and position it in the far background whilst putting a few trees in the foreground to hide the background.