I have a MS chambered for the 6.5x57. Commercial ammo is available from a couple of the banner advertisers, SFRC and Intersurplus.
I believe they also carry cases.
The shoulder on the 6.5x57 is much higher than the 257 Rob and the 7x57 parent case. OAL is very close, depending on which blueprint you look at.
The 257 neck can be sized down easily and a false shoulder created. The case will have to be blown out during a fire forming session.
It's better to just purchase the loaded rounds or new cases and go from there. IMHO.
It's a great cartridge, especially when being used in a strong actions, such as the 98 Mauser, or MS, etc.
2800fps with the 140 grain bullets offered is easily doable without excessive pressures.