orignal cartridge will go for +/- 25 euro a piece, you will find it here but like any 19 .century paper cartridge is rare collector stuff. I had one of this but sold it to another collector years ago.
original bullet was a roundball with barrel diameter so that what you need.
I dont have a ladys rifle but if so I would try to work with 9mm short brass. think the 9mm short brass will fit into the chamber and fall out if you turn the gun. just load a muzzle loader primer back to front into the primer pocket, close with a paper, fill blackpowder into the case and load a .375 roundball in the press. I will try it here and make some pics how to do. imho, pressure must be so low when you turn the gun the brass will fall out because it dont expand with such a blackpowder load.
what is the max. cartridge length the chamber will allow?

update:

dont find 9 mm short brass in my stuff, only a dummy round and base diameter on this Geco case is .371. if it fit into the chamber I dont believe it will expand when fired and stick there after.

to see how it works I take a 9mm para case and cut it from 19 to 17mm than drill the primer pocket so deep a muzzle loader primer would fit.

I have self sticking aluminium foil to close it

the case hold 8 grains BP and if I had a real .375 ball it could be seat in the reloading press.

if a 17 mm case is to much for the chamber it can be cut down more.
I will call this 0,34" PWM-Dreyse