Brass for the 7mm STW is becoming more and more of a problem to find, Winchester would do a run once a year but have ceased as of a year or so ago. The Remington site no longer lists it either, Nosler still makes it if you can find it, BUT it is very pricey.
Probably the best option is to neck down and/or fire form from one of the many other belted cases with a 2.85" length.
The easiest would be the 8mm Rem Mag, parent case for the STW, others would include 300 H&H, 375 H&H, 416 Rem Mag, and even the 375 Weatherby, the later would be a last resort because of the multi-stage sizing, trimming and neck turning.
I have two 30 STW's and hopefully I have enough brass to burn out these barrels and then go to something different, if not will probably try Rem or Norma 300 H&H and just fire form.
Good luck
Dave