Look up AMMUNITION ARTIFFACTS, it is a business that custom makes brass, was just on the site a bit pricey but that seems about the only place to go right now. last year I was in the same situation for my BOB but lucked out after about a three month search and found some.
Just talked to a re-loading buddy about this, Now I have never done this but he says you can make .257 brass from .308 0r 30-06 the trick is heat the neck area of the brass and size it in the .257 die and trim to length. PERHAPS SOMEONE MORE KNOWLEDGABLE WILL JUMP IN ON THIS.