I'm pretty sure that's going to be near impossible to find new .256 Newton brass. However, all is not lost. You would be best served (not to mention significantly cheaper) to make your own .256 Newton brass from necking-down new .270 Winchester brass. They're very close, the Newton just requires necking it down to 6.5mm, and setting the shoulder back using the proper .256 Newton full-length sizing die, and then trimming the cases to 2.450".
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