Try the website hoseeker.net. I assume you’re talking about the 89’ autorack kits. I just checked, and they have some images of the instructions listed, but doesn’t seem to have a specific part number associated with it. The models are series 4800, and the roof is part #3 on the diagram, if that helps.
As an alternative, you could probably easily scratchbuild a new roof using corrugated sheet styrene. May even be cheaper than ordering a replacement roof part from Walthers.
Or if you have another autorack model, you could use some aluminum foil to make a copy of the roof by burnishing it over the top to get the corrugation pattern, then cut it out and glue it to some styrene sheet. You wouldn’t even have to paint it! [:D]
Walthers Auto Racks are hard to find. The only ones I found were on E-Bay and not cheap. I think the only Auto Racks made now are Atlas and Intermountain and they are backordered or sold out. Must be a popular rolling stock item.