Check with Graf&Sons. www.grafs.com
They are showing Winchester 7x57 brass in stock at the moment.
That's one of the cartridges that Winchester (or whoever they farm it out too) only does short annual run for and those short runs usually disappear pretty quick...
I picked up a couple bags of 7x57 Remington brass recently from Midway to reform into 6.5x57 for friend's project gun. A little cheaper than WW brass but I've noticed lately that Remington brass is getting a little sloppy too...I had to cull about 3 bad cases out of that lot of 100. Folded necks on all three.