In my experience the best way to find wood is city ruins. Just walk on the map until you find some little light brownish squares which resamble city ruins (for example a few squares left and down from NCR near the Brotherhood of Steel and the desert). The big city area around the south and south west coast works too of course.Enter the encounter by clicking on your position arrow on the map. You will encounter different settings of the city ruins map-type. One or two of them have choppable trees. If you don't get one of those maps at your first try, just leave to worldmap and enter again - you don't have to move on the world map for a different encounter-map.You can also find tons of junk in those maps, which is used to craft other items and weapons.
