OP asked for black powder 38 S&W. The Buffalo Bore 38S&W is loaded pretty hot and would most likely damage if not blow up a black-powder 38 S&W revolver.
I have not seen anyone loading black powder 38 S&W commercially.
I looked again and it is out of stock at this time, but perhaps reread my post and check out the link
For the money they seem to get these days for a box of 38 S&W of any flavor, Id suggest getting into reloading if you want to shoot a lot of it.
What I load costs me about $7 a box, and thats for something you wont normally find anywhere. Matts sells the same thing I load, loaded for $40 a box of 50. Hes the only one Ive seen selling it too.
Plain old 38 S&W goes for $35-40 a box of 50 around here, if you can find someone who carries it. Online is a better source.
If you reload, brass and bullets are readily available, and you can have a lot more lattitude in what you load. Its also a LOT cheaper too. I load it as 38/200 for my Enfield and Victory's.
If you dont reload "yet", it would be an easy round to start with.
I had a S&W 4th Model top break that was of the black powder era. I loaded for it and used black powder. Its another one that will cost you an arm and a leg if youre buying factory.
For what a box of factory would have cost me, I got a set of dies, 100 pieces of brass and 100 bullets for $20 more. A pound of black powder would probably last you years.
Black powder is a little different than smoleless to load, but its still easy.
This is what I was going to recommend, Matt's and the 150 grain bullet in the biggest diameter possible. These top breaks often have big throats, one of mine for my HR .32 is .316"OP asked for black powder 38 S&W. The Buffalo Bore 38S&W is loaded pretty hot and would most likely damage if not blow up a black-powder 38 S&W revolver.
I have not seen anyone loading black powder 38 S&W commercially.
I don't necessarily advocate this but I have shot both PPU, Magtech and Remington 38 S&W commercial ammo in my second model Iver Johnson safety automatic that is technically a black-powder revolver without issue. Those three commercial loads are all pretty anemic even by blackpowder standards of the day. YMMV and at your own risk on this for sure.
Matt's bullet makes a few correct diameter (.361) bullet for 38S&W reloading. Most of them are 200gr bullets for creating replicas of the 38/200 used in Webley and Enfield revolvers but his 150gr bullet would work well in black powder reloads if you willing to reload them yourself.