>>64118982
You'll have to handload them. Need 2f powder, dont use pyrodex. Use 80 grains of powder and 1-1/8 ounces of shot for a 12 bore. 70 grains of powder and 1 ounce of shot for a 20 bore. Lead shot obviously, dont use steel BBs. Easy to load and fun to shoot.
Beware, your gun might have 2-1/2" chambers, using 2-3/4" shells will fuck it up.

As the other anon said, minishells will probably be safe pressure levels for your gun if you want off the shelf ammo.