The Firearm Blog's recent pieces on early "high capacity" repeaters had a picture of a Miegs rifle which, while interesting, would be no more than an extremely rare prototypical footnote if it hadn't obviously influenced the later rifles built by the Evans Repeating Rifle Company out of Maine, which were a qualified commercial success.
The Evans were manufactured from 1873 to 1879, and roughly fifteen thousand of the helical-magazine repeaters found buyers during that stretch of time, and were even endorsed by "Buffalo Bill". As a result, they're not terribly uncommon at gun shows today if you know where to look, and while premium examples bring premium prices, serviceable shooters can be had for well under a grand. The .44 Evans cartridge hasn't been commercially loaded for almost a hundred years, but the black powder rounds can be formed by cutting down .303 Savage brass.
Of course, "high capacity" is relative to the time and place: While the user of a later Evans, which due to its longer cartridges held six fewer rounds than the early models, had twenty-eight times as many rounds on tap as a contemporary U.S. soldier (who used a "Trapdoor" Springfield), he only had twice the magazine capacity of a Swiss private armed with a Gew. 1869 Vetterli.