Why not make one of your own? Over the years I have owned any number of sometimes weird caliber arms and, like you, I wanted to display an example of what they fired, along with the arm. After purchasing what I assumed were purportedly original cartridges, sometimes at a stiff price, I realized that I could do a pretty good job of making these cartridges myself. While I understood that the cartridge was of recent manufacture I suspect that some of the early cartridges I purchased were put together long after the historical period they were supposed to be from. By the way, so that no one ever tried to load and fire the thing with the display cartridge, I used a dummy powder charge.