You can search for dead trees, or pick up tree branches that have been snapped off due to storms or other natural means and take what has naturally fallen on the ground. That way you are not hurting a healthy living tree, nor stealing from anyone either. If you know what you are doing, you can prune some sucker branches, but only if you know how to prune and where. There is a wrong way and a right way to pruning. I also second the advice to get permission from the landowner to see if you can find fallen branches or to prune, just in case they catch you on their property, dislike it and charge you with trespassing or vandalism, or worse.