RWS brass is available from:
It isn't inexpensive but the quality is certainly the very best. You have to determine if the quality (and price) is really required for your application. The other thing to consider is, do you really need 100+ pcs. or will fewer pieces work for your needs? I buy 40 pcs. for each of my rifles chambered in a European cartridge that RWS makes brass for. For the rest, I use Remington, Winchester, Lapua or Norma, depending on what's available and what the need is. I certainly wouldn't buy RWS to feed a varmint rifle.
Several years ago, I bought some Hirtenberger 8 x 68 and 6.5 x 68 brass from OWS and found to my chagrin, that the cases weren't of a very high quality, but they were less expensive than the RWS. They lasted 4 reloads for hunting rifles (not excessive hot loads) while some of the RWS that I now have is going strong after twice as many. OWS is still listing some RWS brass. Some 6.5 x 55 is on sale.
I can also get RWS primers through a local source. They are very consistant lot to lot but I can accomplish the same thing with Federal Gold Medal Match at less cost.
The only listing that I know of for 338-378 WBY is Norma for Weatherby. Huntingtons carries it also.