You will be pretty limited on what you can get right now. ADG which is the best just released 300 WSM brass but it went fast. There may be a few out there but good luck. Your next best option is Norma. As of right now Unknown Munitions has 300 pieces in stock. They are good to go as far as a seller. I want to say its $119/50.
Thanks for the “vote of confidence” in unknown munitions. There’s a lot of bad sellers online and it’s hard to know who’s good when the well known ones are out of stock.
I’ve also thought about just buying Norma loaded ammo and using the once fired for load development. It’s not much more than the brass itself. I’ve got to break in a barrel anyway and it looks like pretty good stuff.
The loaded Norma ammo is a good option. Otherwise I'd definitely recommend unknown munitions they have been fantastic to work with. Sometimes just having them load it makes a lot of sense. Especially for the first batch and can have them put about any bullet in it you want