There's no such thing as a "belted Newton" cartridge.
Charles Newton didn't do belts.
You're going to find it almost to totally impossible to find ammunition, brass, or loading information for this cartridge.
Both it and the .35 Newton have been obsolete for decades.
The last run of brass was made just after World War II when Fred Huntington of Speer fame made some.
The story goes that Fred Adolph, a well known gunsmith at the time, contracted with Newton to design the .30 and the .35 as proprietary rounds. When Newton started his "Buffalo Rifle" company he took on the two rounds under his own name.
The biggest problem with the rifles are that they are simply too lightly stocked for such powerful cartridges.
The .35s in particular had a VERY bad reputation of shattering through the wrist under recoil.
I'm in Pennsylvania right now dealing with the aftermath of the death of my Father, but when I get home and get access to my reference books I'll see if there are any common cartridges that can be converted to the .30 Newton.
If you don't hear from me within a week, send me a private message.