Take a look at this link:

https://forum.cartridgecollectors.org/t/25-roberts-257-roberts/6587

It should answer your questions. From the pictures, it is obvious that you will not likely find any reliable brass for this gun. The factory Roberts brass should not chamber in it unless it has been reamed at sometime in it's past. Best bet would be to have dies made from a chamber cast or (not likely) find a set. Then form from readily available 7x57 brass.

Rechambering to factory would be the move as a user, but that decision has many financial and use upsides and downsides.