There is no reason to make or use nickel-plated cases in these calibres.
Does anyone know if you can get nickle plated 50 cal or 338 lapua cases anywhere ?
My question would be , why?
You are asking that on CGN? I would suspect the first answer would be 'they look cool'. Second I suspect would be 'cuz'.
IIRC Hitech made some years ago, plus there were some fluted dummies with a nickle case. Can't think of any on the market now though. You might find the odd one a collectors show etc.
Sorry, I tend to think more of practicality and usefulness. Nickle plating does separate from the brass substrate that the casings are made of even with little calibers. Given how much the 50 BMG casings typically "grow" with every firing, nickle plating would be a real bad idea. I would suspect that after 2 or 3 firings the nickle plating would start to "flake" off the casing and get embedded into the chamber of the rifle, which of course is less than a desirable effect.