New Camper sounds right. I haven't seen the stuff since the 80's, and what I do remember was John Wayne commemorative stuff (was a kid then and might be wrong).
There might be some limited runs, but you've got to be at the right place at the right time.
Sounds like if you don't handload, you might have to start.
Handloaders Manual Of Cartridge Conversions says you've got to use either factory brass, or 375 Win to make that critter up. A tapered case that isn't bottleneck, but looks more triangular in profile.