I had considered using a mist unit in one of my launches as a sort of ' faux' boiler as they work well. I could only find a horizontal boiler shaped unit from Model Boat Bits here in the UK. They cost around £50 which isn't cheap, but certainly cheaper than a boiler. I did find a vertical example but they don't seem to be produced anymore.
If you weren't bothered about steam coming out of it, it should be easy enough to mock up a boiler out of a tin or similar with mahogany lagging - or make up a cylinder from styrene sheet.
Many steam launches had the engine covered with a wooden casing, ie the Windermere launches, and that would also be a way of disguising fact that you didn't have a real steam engine, although reading your post again, it sounds as though you intend to use a steam engine turned by an electric motor.