The Royal Sussex Regiment (35th Regiment) pointed out a couple of great sources for firearms, and one of them is kind of out of the ordinary. This group actually does two impressions, one is based in the American War for Independence and the other is around the time of the Zulu Wars. This comes from the need to outfit the unit with Martini-Henry rifles for the Zulu War impressions.
One source of all places is Atlanta Cutlery Corporation. If you thought like I did, this company doesn't seem to be a vendor for antique rifles. But I was wrong. They have the Martini-Henry rifle in limited stock. They also have the Snider (Enfield percussion to trapdoor) conversion too.
The next vendor they recommend is International Military Antiques. This place caters to several time periods, sell some reproductions (insert my usual disclaimer here), and is an interesting surfing spot. They also sport the Martini-Henry at about the same price as ACC.