I thought, "That can't be right" but when I searched, you are correct (as of 3-24-19):
Christensen Arms was a NO
Proof Research was a NO
BSF Barrels was a NO
CarbonSix was a NO
CarbonSix makes it seem like you can 'make your own' but then they don't list .323 (8mm) as an option. Christensen Arms makes it sound like they'll make you a barrel blank and then you can have it drilled and rifled to whatever you want, but again, they don't show .323 (8mm) as a choice. Still, I think I'd e-mail or call them. If they did 8mm before and just don't list it now, perhaps they'd do a one off for you since they'd still have the tools to make it happen? If they won't do that, I wonder if they would at least sell you a carbon-wrapped barrel blank that you could then send it off to be drilled and rifled in 8mm by someone else.
If no carbon wrapped barrel is possible, perhaps a steel barrel with heavy fluting? There is a calculator you can use here to see what the final weight would be:
http://riflebarrels.com/computer-software/