I've got a couple of Sniders that I cast my own bullets for. NO, I don't really enjoy casting enough to want to do it for anyone else.
Yes, there is a market for the bullets, albeit small.
At first, I tried some solid cast bullets that were a few thou larger than the diameter of my bores. This created problems because the chamber was to small in diameter to accept those bullets and I had to thin the case necks so the cases would fit. Just another pain in the butt.
Solid bullets are not the answer.
You need hollow base bullets with a clay (auto body Bondo) plug. They don't necessarily have to be soft lead either. The British and colonies did use pure soft lead though. I haven't found an original round that is in bad enough shape to take down, so I can't tell you whether or not they had plugs in the skirts of their bullets. I know they used hollow based bullets.
Their reason for doing so is simple. When the cartridge is fired, the pressure from the burning powder pushes the plug into the base of the bullet and causes the skirts to expand into the rifling, thereby giving good contact as well as a good seal for consistency.
The hollow base mold is still available for the 577 diameter cartridge.
I would also like to find molds for the 38 Webley and 455 Webley cartridges, with hollow base capacity. Sadly, no one makes them anymore, not even by special order.
One thing to remember, the old antique class firearms were made one at a time, by hand labor. Very few of the parts are interchangeable. Some of the later made pieces, like the Webley pistols do have some parts that are interchangeable but not many and even those are hand fitted.
All of the bores are different from one another. They are close enough but it would have been a nightmare for the manufacturers at the time to make the excellent barrels that were available by the turn of the 20th century. Even those were not consistent but much better than those on the old black powder firearms made for military purposes.
There are lots of different methods to get brass. You can buy Bertram, which is made in Australia and can be found on the internet. They come with modern, single hole primer pockets and will reload for a long time. Most people either just use FFG or FG black powder but some use a duplex load including Hs6 or some other fast pistol powder over the primer and a compressed load of black powder. Black powder really likes to be compressed or it will not ignite in a SAFE consistent manner. If there is to much air in the case, it literally will cause a small detonation and maybe ruin your firearm and injure you.
Go to the Snider Collector sites and there will be a ton of information for your searches.