280 Ross hasn't been made for roughly 100 years. Might be makeable out of .275 H&H flanged. That'd be pricey.
Rim diameter was .560".
http://stevespages.com/jpg/cd280ross.jpg
Quality Cartridge says they'll do a run when sufficient back orders exist at $71.97 per 20.
These guys(up here) say they have proper diameter bullets, but they ain't cheap.
~ $65.00Cdn. per 50.
https://www.summachsports.com/
You'd best sit for this.
RCBS wants $510.95 for special order case forming dies.
You'll probably want this.
http://ps-2.kev009.com/ohlandl/Cast_..._280_Ross.html