Your best bet would be to reload your own, both for the cost savings and for being able to select loading formulas like BP or very light smokeless loadings that are a bit off the usual loading you'll find in commercial ammo.
Follow all the safety rules and published loading data always.
For BP you have to fill the cartridge right up to where the bullet seats, leaving no air gap. Not sure many places still carry it, but I found some years ago at Williams up in Port Perry. Note that BP burns more violently than the modern stuff, so actually pay attention to all the safety rules when you're handling it.
For very light smokeless loadings, Trail Boss seems to be the powder of choice, and stay at the recommended starting load.