1. Throughout Uralia (Okytabrya (sp?) I believe has one... throughout Uralia though there are some)
2. User's private homes (ask on Global Chat), or various housing blocks
3. Lowland Slough, Scribe's Weald, Eastern Approach, throughout Ancestral Lands (Choru, Zhambu, Baqala, etc)
4. West Spice, throughout Ix
There are applications to find these, but I don't know it offhand D: Hopefully someone can post the link!
For a gas tree try the marsh streets (Chakra Phool, Shimla Mirch).
For wood tree walk around residential blocks (where I live for instance). I tend to fine some on every other block.
For a paper tree go to the ancestral lands (Baqala, Choru).
For spice trees go to Ix, I hear they have so much spice it flows.