This may not fly everywhere, but down here in the low countries gardening/plant stores (big ones, usually out in the countryside) will have big x-mas displays going. Right after the holiday some of them will sell the trees used in them by the bag for dirt cheap. I was lucky to pick some up, ranging from 3 to 12 inches IIRC. Your foliage trick looks fantastic, I really should do that with mine it looks like a big upgrade over the bare spines IMHO. Flock/foam has that right level of fuzziness to it that your mind fills in the details of a tree at a distance, while the simple spine limb doesnt.
I would also note that at 5cm they are fine for 6mm 1/300 games as well! A 15 meter tall pine tree is quite tame in fact, fully grown pines run the 30 to 50 meter range!