Bertram .400/.360 WR brass @ Grafs https://www.grafs.com/retail/catalog/category/categoryId/827?
....or “they” say 9.3x74 can be resized to work.

Hawk bullets can work with you to make .360” 300 grain http://hawkbullets.com
Probably best slug the bore?
Groove diameter bigger than .360 but less than .366 you can get Corbin to make you a "Bullet Reduction Die" that can squeeze a .366 bullet down as much as .006"

Dies? I suppose get in line at CH4D or order from the guys in Oz as they can ship dies to the USA.

- Mike