Styrene vs wood also depends on how old your prototype is. New, or freshly painted, wood cars show little, if any, grain, making styrene the better choice in N scale. If your prototype is badly in need of shopping, wood is certainly the way to go.
Most scale lumber has too much grain for N scale smooth boards.
If you really want wood, Kapler Mill and Lumber Co. has a good selection of both dimensional and N scale:
http://www.kapplerusa.com/y2k/kp-main.htmI've never bought from them, but have used their products. I like the disclaimer on their home page, in bright red letters:
"Please note that we are not a supplier of lumber for the building industry, so if you are looking for cedar decking to finish your home project, we are not the ones to call."