You might try a mag from the 308 family of rounds. Pretty close, I think it uses the same one as well as the 300 Savage.
In dimensions the 30-30 is not all that close to the 35 Remington. The 35 Reminton is based on a shortened 30-06 case and is rimless, the 30-30 is based on the 38-55 necked down and is a rimmed case.