I’ve always collected most of my rocks and wood from outdoors, and many of my plants as well! I dig up roots of smaller trees or bushes, because the root structures look better than the branches IMO. Always age your wood for a few weeks if you are collecting it live though; I boil it and then let it sit in water outside for about 2 weeks. There are a few species of wood that can secrete poisonous sap and such, so do a little research on what kind of trees are in your area. For rocks, I like getting river rocks from local streams; I dry them out in the sun for a few days, rinse them a couple of times, and then they are good to go. I’ve even collected a few species of ludwigia and some parrot’s feather from local ponds, and what I think might be some sort of hygrophilia, but I’m not exactly sure. I always give them a quick dip in a dilute bleach solution to kill off any stow aways, and then rinse thoroughly. The only man made thing I’ve put in aquariums from outside was shards from terra-cotta pots and bricks; I’ve had no problems with either.