Go two spaces left of the Obsidian Mansion, then head up two. You should be at the giant tree. It gives a random tree seed (twinkle/sparkle ones make trees that you can cut for loads of lumber). There'll be stumps for you to cut around it too, a minimum of about 12 each day?
There're at least 3 smaller fields out in the world area. I scrapped up the last bits I needed for my Pot from those. If you have the airship license, you can instantly jump to those places (if you've already visited them). --- http://i.imgur.com/2vCvvlm.jpg http://i.imgur.com/6Wu30zR.jpg
- On your own fields (if you haven't tilled them all). - Pretty much everywhere on the overworld. The further you progress into the game, the more common lumber and material stone are going to become. --- It could be worse. I could be a Prinny.