This place carries brass and dies, out of stock now, but worth a try. https://www.buffaloarms.com/

Other web sources say they use .348 Win brass as the rim is closer. The 45-70 the rim thickness and diameter need to be reduced. Cast Bullets Journal had an entire article on the cartridge in 2004.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli