While difficult to locate, 7WSM brass does appear from time to time. I just purchased another 330 - new Win 7WSM cases from a poster here on LRH. That should last for about 2,000 rounds of reloads.
With the brass I have on hand, I'm good for the next rebarrel or two. I really like the 7WSM and have given up on my earlier plan of rebarreling/rechambering to 7-300WSM even though that wasn't a bad idea.