Thanks for the reference, guys! I looked and made a few calls, and I can not find any 257 Roberts brass. But.... I can find a lot of 7x57 Mauser brass to neck it down to 257 Roberts. It's a simple process running the 7x57 brass through a 257 Roberts sizing die. I can get Norma, Nosler, and Prvi brass. I'd recommend the Norma brass for the higher quality to neck it down from.
If you would like some pricing and availability, send me a PM here or email me "chad at dallasreloads dot com". Let me know what bullet you would like.
If you guys are local, you can bring the rifle to me and I can chamber check and seat the bullet to your chamber for better fitting. Just let me know!