Hudson & Allen Studio's sell bags of the aforementioned birch seeds as "Fall Black Oak Leaves". They also sell something called "Pine Boughs", which are basicly cured bits of moss that look like 1/35-1/72 scale pine boughs. This is amongst a wide range of other museum-quality scenic accessories. The only place that I've found that has a webpage of their product line is Michigan Toy Soldier Co. (
their Hudson & Allen scenics page), though Brookhurst and several other companies carry their product as well as many a very well stocked local model hobby shop.
Another place to check out, and very easy to do business with, is
Scenic Express, who have just about every scenic accessory under the sun. You'll be looking for
Leaf Flake & Foilage, specifically Noch's Leaf Flake, though, as you can see, there's plenty others there too. If the pics aren't big enough, I'd suggest you moseying on down to the biggest and most well stocked railroad/scale model hobby shop in your area, chances are they'll carry some of the same products.