How do I get Silverwood saplings?

Silverwood trees apparently purify the area around them, and have their own aura nodes. It seems like a no-brainer to plant a couple of them around my base to offset the damage my inept thaumaturgical tinkering is doing.

However, I haven't been able to get any silverwood saplings at all. The Thaumonomicon has this to say:

a silverwood sapling [is] a prize beyond measure.

So I assume it's not as easy as cutting down silverwood trees. Doing that yields nothing at all, aside from the wood.

Is there something special I have to do to get these saplings, or is it even possible at all? I know the saplings exist as items, but that could just be for server ops to spawn in.

They come from the leaves of silverwood trees, the same as any other tree. They are, however, extremely rare, so maybe expect one from a whole tree.

This is to balance the fact that plating one makes an Aura node near (or inside) the tree.

I've been told you can get one sapling out of maybe every three trees you cut down. Seems like it would be more efficient to just build your base around the tree.