There is a little thing called a "police action" ("war" is no longer politically correct...) going on and they get first priority... oh wellllllllllll The "middle east?"
#b). You can google .257 W. and see what you get... "once fired"
This is not the most widely used cartridge around and finding once fired??? LUCK.
#c). The other option, if you are not some "purist" /"nut" is to substitute. I had one customer finding him .300 Weatherby brass. No, blowing out .300 H&H was not acceptable. His $$$... You can make the case from a long list of other "belted" brass. Obviously, the original was the .300 H&H Mag. Another less than common round these days. At the same time, I think you can neck down .264 WCF Mags and come awfully close. Might be a trifle short and you might need to reform the shoulder. Another less than common case. Now the 7mm Rem Magnum is close too. This should be available in "once fired." This should work well if you are not shooting competition.
#d). Shotgun News [shotgunnews.com] hard copy has dealers in brass, once fired. As I said in "a" this is not the best time to be looking, but no telling what might turn up. From there it is the auction sites. LUCK. Happy New Year.