Buffalo Arms has AMMO
https://www.buffaloarms.com/25-25-stevens-black-powder-ammo-box-of-20-amo2525stevensb

Best brass would be RMC
http://www.rockymountaincartridge.com/index_htm_files/RMC%20Product%20List.pdf

https://www.grafs.com/retail/catalog/product/productId/1116

86 grain lead bullet, 5.5 grains of Unique

Use the gun as intended and if not usable for this purpose, get
a different gun.

Welcome on your first posting

Mike