Any type of wood is fine in my opinion as long as it isn't a resinous or fir tree (i.e. pine and things of that sort). If the wood was fine for an aquarium its going to be fine for your snakes. If you're getting wood from outside I'd say remove the bark and bake it well to make sure you don't give your snakes any unwanted roommatesalso make sure you collect the wood from an area where there are no pesticides used and that there weren't any used in the vicinity.