If you can find something that's been soaking in water for a while, that's probably good. Otherwise, try to find something that's been well-weathered, repeatedly exposed to rain and sun.
If you think a piece of wood is questionable, try scratching/breaking a piece of it off and see if you can smell any odors. 'Earthy' or 'wood' smell is fine, but if it's more like resin, pine, cedar, or something else, you probably want to avoid it, or at least give it a pretty thorough soaking.
I think root pieces tend to look better then branches, but that's a personal preference, and it all depends on the specific piece of wood involved.