Your best chances are Ebay, Gunbroker or similar auction sites. The 284 is an odd cartridge and I am wondering if one of the short magnum magazines would work. I know the 243,308 and 358 all share the same mag. The 284 is not a 308 based cartridge and I cant think of one it matches. The case measures .500 while the head measures .473. The 308 is .473 and .470 for the case. The loaded oal is the same as the 308 at 2.800 so it is definitely a short action. It is possible that the 308 would work with a touch up on the lips with a dremel.