I have used quite a number of nickel 45-70 cases over the years but haven't seen it available for a long time. I question whether or not it's still being manufactured. Starline and Rem had nickel cases IIRC. My last purchase was 250 pieces from Starline but it was plain brass, nickel was not an option at the time.
If you only needed a few for a special purpose, I could dig some up but don't have any quantity to sell.