In 'Merica, firearms designed to use black powder rather than smokeless are not considered firearms. Thus, so long as you're 18 years of age and have the money, black powder weapons are rather easy to get.
Also, anything manufactured before.. 1899, I think, is considered an antique, thus can bypass the normal firearms laws.
Damn shame there's nothing that can really be done for the Napoleonic Wars here. I mean, there's the War of 1812 stuff, but that's all north of where I am, so it's too far to go.. (My complaints end here, I swear.)