I would retry NWSL. They normally have suberb customer service. Might do better with a phone call than an e-mail - many manufacturers have their spam blockers set pretty high and/or give low priority to e-mail. Also, places like Caboose Hobbies may have the NWSL parts in stock - I've bought NWSL motorizing kits for my Keystone Shay from Caboose Hobbies before.
Parts supply for items like the MDC Shay have likely dried up at Horizon Hobbies. If/when production ever resumes, parts may become available then. I imagine the Shay was not a money-maker for Roundhouse, even though it filled a needed niche in the hobby. Way too many service and support calls from people who had problems with both the kits and RTR. As far as I can tell, the RTR were just fairly unskilled assembled versions of the kits, so the deisgn and manufacturing problems of the kits, which if not corrected during assembly, would also infect the RTR. The careful kit builder could/would correct these problems during assembly. For these reasons, Horizon Hobby will probably think long and hard before resuming production.
Again, the Roundhouse Shay was/is not a simple locmotive to get running well, but it can be done. Jeff Johnson's book, "The MDC Shay Handbook" is a tremendous aid in understanding these models. Until Bachmann came out with its Shay model a year or 2 ago the Roundhouse and the Keystone Shays (the latter come unpowered, you have to motorize it yourself) were the only under $200 Shay locomotives available.
yours in gearing