I have used Bachman stock cars for some time. They have wood roofs and sides, but with some weathering, can look quite acceptable even behind more modern locos. The BMs aren't too expensive. I also put a self contained sound system in one of mine, so it moos, too. I run the stock cars with a couple of MTH Armour and Swift Reefers to make my "Where's the Beef," train.
This is a photo of a 1:22.5 Bachmann stockcar (not the new 1:20 scale). Except for repainting, relettering and weathering it is unaltered (I did lower the end roof platforms a bit). These cars are inexpensive and quite durable enough to survive being left outdoors much of the time.
Stock trains are on the agenda for the POC RR as soon as it's completed a bit more. They will be seasonal with cattle and sheep being transported to Spring pasture and brought out in the Fall. Other traffic the rest of the year will be individual cars. Of course here a long train is about 10 cars....