Treblig, I am unaware of anyplace where you can buy .32 S&W loaded with BP. But I do load my own with blackpowder and smokeless, I have a couple .32's.
The .32 Long holds a lot more powder as you can imagine than the .32 S&W, in case you are confusing the two. And one of my guns shoots .32 Longs, so that's what I feed it. But if your converter shoots only shorts, your best bet would be either handloading or buying them from somebody who handloads in that caliber.
Triple Seven is the best powder for this application in old guns and CAS, but I've loaded mine with a .22 short case of Hodgedon Titewad. Loaded with BP, they are neat rounds. Loaded with smokeless, they are a very mild and accurate load.
Here's some inspiration for you. This is what 10 grains of BP will do behind a wadcutter touched off by a CCI small pistol primer in Magtech brass. This was shot one handed at about 30 feet, in single action. To be fair, I did fine tune the trigger. I unraveled those cartridges this morning (I had a heck of a day yesterday, BP is my cure for stress)
I didn't put any target on there, I just shot a hole in it and then proceeded to shoot at the hole.
The barrel length is 3", about the same as a 49' pocket. The sights remind me of a cap n' ball Colt. I reckon if you hold her still enough you'll be able to do about the same or even better. Keep in mind that if you are considering using/carrying it for defense that the .32 Short is anemic. Accuracy is it's only virtue, and the fact that the report is delightfully quiet. The sound signature doesn't travel too far, and when loaded with smokeless it isn't as loud as a standard velocity .22 Long Rifle. Yes, it will shoot through 1/2" of pine which is to FBI lethality specs. Would I carry it as my primary? Nope. Is it better than pocket sand or the middle finger? And can it kill somebody? Definitely.
If you are thinking about getting into reloading, pick up some FFF or FFFF powder, or some Titewad and some small pistol primers, Midway USA should have all you need to make ye' some cartridges.