As others have said, be on the lookout for .257 roberts or 7x57 mauser brass as well. It can be resized and work just fine. Or on the off chance that you find .22-6mm or .224 Texas trophy hunter brass, those can also be resized to work.
Or you could just have a .243 win like
everybody else 
I have about 500 pieces of 6mm brass to feed my three rifles chambered for it.
Should last me a few years. Sure am glad I got it before this shortage