There are lots of wood (usually styrene simulations) cars available.
Note: most of mine have been modified with extra details, and most are custom painted.
Here are a few examples:
Train Miniature (now Walthers) 40' single sheathed boxcar
Athearn 40' wood sheathed refrigerator car
Red Caboose 40' wood sheathed refrigerator car
Accurail 40' USRA double sheathed boxcar
Accurail 40' single sheathed 9 panel boxcar
Accurail 40' single sheathed 6 panel boxcar (at right)
Proto1000 36' Fowler Patent boxcar
Train Miniature (now Walthers) 40' USRA double sheathed boxcar
Intermountain 40' composite gondola
LifeLike (not Proto) 36' double sheathed refrigerator car
Athearn 50' double sheathed express reefer
Train Miniature (now Walthers) 40' double sheathed refrigerator car
Walthers (the car to the right) 50' single sheathed doubledoor boxcar
Accurail 40' double sheathed refrigerator car
Tichy 50' War Emergency composite gondola (the Wabash car)
MDC (Roundhouse) 36' double sheathed boxcar (made from a 36' truss rod reefer)
Tyco 40' double sheathed refrigerator car (the Grand Valley Export car, to the right)
MDC (Roundhouse) 80' wood combine (also available, a Pullman, diner, and observation)
MDC (Roundhouse) 55' postal/baggage
MDC (Roundhouse) 55' combine
MDC (Roundhouse) 55' coach
Sylvan wood caboose (resin kit)
Walthers also offers several different styles of wooden cabooses. Hope this helps with your search.
Wayne