According to this site ... http://www.chuckhawks.com/38-40win.htm a minimum charge of 6.4 grains of Unique with a 180 grain cast lead bullet is good for a MV of 776 fps from a revolver or 1162 fps from a rifle.
Winchester Super X 38-40 Win 180Gr SP Ammunition has a MV of 1160 fps so I presume its already low velocity?
Gagnon Sports has 2 boxes listed. http://www.gagnonsports.com/index.php?product=WIN-X3840
You could try Ellwood Epps too.
You should contact Rusty Wood Trading Company. They are in BC too. He sells ammo for antique lever guns , as well as does custom loading.