A cheaper way is to buy the tree armateurs and then buy some tree foliage . They are not difficult to build and i have built many for the model railway. In my opinion they are a little too fragile for wargaming but brilliant for dioramas.
Anywhere that sells woodland scenics, really but Hobbycraft has it. It's like a PVA white glue in a copydex type bottle with a brush. Paint it on where you want foliage or scatter, leave to dry until it goes clear and you find it's really tacky. The benifit is that if your trees start to moul you can strip off the material leaving the hob-e-tack on and re sprinkle the old stuff back on. If it loses stick you can add more or run it under a tap.