I have also never seen those trees for sale in guild stores when I was buying clockwork stuff, but YMMV.
If that doesn't suffice, your best bet is to open a ticket with Support. I usually have good luck with the in game Ask For Help option. That's about the only way you'll get an answer straight from ZOS.
Occasionally, things drop that are, supposedly, not in the game at all (and sometimes, not even in the Crown Store!).
For example, I have a Velothi Painting, Classic Geyser (from a drop) that you can't buy from the Crown Store and, according to ESO Fashion and Google in general, doesn't appear to drop in the game, at all, in that size:
Therefore, I think it's difficult to say something definitely isn't available; but, it might be so very rare that (as far as being able to reliably acquire it) it might as well not be.