Just a heads up, all my local Houston Game Stop Stores have received their initial shipments of their Exclusive The Black Series 6-Inch Purge Stormtroopers this week, but each store only received enough figures for those that preordered the Jedi: Fallen Order Game Bundles that are to be released starting tomorrow night (11/14). I also called a few stores on the East and West Coast and they said the same. Each store said they will not have any individual figures for sale this week, unless someone cancels their Jedi: Fallen Order Game Bundle Preorder.
Corporate and store Managers that I spoke with did say these will be for sale individually after the Game Launch as per their 2019 Holiday Gift Guide (Page 32) and their Game Stop TV Ads in stores.
So looks like you will have to be a bit patient and hopefully they will receive more shipments of this figure in the coming weeks.
As for the availability on GameStop.com, I don't have a 100% confirmed answer yet, so will just have to check their site once in a while.