A couple of evenings ago, I got out my very nice Lifelike C&O Berkshire and sent it around the layout for about a half hour. Upon repacking it, I noticed the valve gear on the right side was gone. I've searched everywhere with no luck. It is entirely possible it went missing from running previously without notice until this time. I have near zero confidence in getting the part from Walthers. Does anyone have one of these locomotives they have parted out?
You might try placed a Want ad in our Swap Meet section. A good number of folks browse through there, daily.
Steel or brass? If steel, find a big magnet, like from a big speaker, set it in a depressed centre flat, haul it around the layout. You might be surprised the stuff you come up with. Used to do that...cleaned up spikes, coupler springs, screws, bolts.
I was able to get a few replacement parts from Walthers. I believe they have some returned units that they can remove parts from.
The action of the drivers may have slung the part a slight distance from the track so I would widen my search some. Also did you check the packing in the box good? I have found parts occasionally sandwiched under the foam inserts. Otherwise good luck with getting anything out of Walthers. Lifelike when they were independent had good service, can't say that now but you might get lucky.