Where does one get ANY ammo? I don't know of a source for any quantity at a reasonable price. Someone once wrote (I think on this site) that .41 Magnum cases can be trimmed, but that is not true as the .41 Magnum case is .03" too large in diameter for the .41 Colt chamber.
That was a very popular round in the late 1800's and early 1900's. I think it was the third most common round in the SAA Colt, after .45 Colt and .44-40, and it was used in many other Colt models like the one pictured. In fact for many years that Colt frame was called the ".41 frame."
Jim