Yep, .30-40 Krag. AKA .30 Army. There's no ammo or brass available except 'seasonally' from Winchester and Remington with no mention of when that season is and Stateside. According to Midway USA(no exports), Remington is in the middle of a season now. Of course, that doesn't mean you can get any here.
It can be made out of .303 British. Mind you, you'd need dies and those are special order. Otherwise, it's just running the .303 case through the Krag sizer die. Rim will be a bit smaller and the neck a bit shorter, but neither will be off enough to matter.