I've used .300 Weatherby, .300 H&H, .375 H&H, and 8mm Rem. Mag brass to form 7STW. .300 H&H ends up with a short neck but they all work. The easiest is 8mm Rem. It's like necking a .300wm to 7mm
.300wm brass is hard to come by as well if your stuck on a brand but it's much easier to come by then 7STW or 8mm Rem Mag. If you open to using what you can get then 300wm is much easier to come by and will always be.