Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Certainly the don't show up often, but I would think the eBay would be a good place to keep tabs on, as well as some of the Facebook steam groups. Other than that, steam shows and gatherings would likely be a fair bet, though as you say, the field is rather sparsely populated, so you'd have to expect to come away empty handed most of the time.

Best of luck in your search and happy steaming in the meantime!!!