That's a Mk.IV rifle. I own one of those plus a couple Mk.II rifles which are closer to what was used in the movie "Zulu"

You can buy those from International Military Antiques or Atlanta Cutlery and they will ship it to your door.

Ammo can be an issue as .577/.450 Martini Henry is expensive and hard to find. I've got some Ten-X solid brass that won't fit all three of my rifles....however I've been converting 24ga. CBC shot shells to .577/.450 and these work great!