Are you looking to decorate in a way that fits in with the actual production design of the movies? Or are you wanting a bunch of Harry Potter logo/graphic merch? Nothing at places like pottery barn or other retailers will ever give you that authentic in-universe feel. For instance, you'll never see Harry with a pillow that says "Always" on it or a Golden Snitch LED wall light in the films.

You mention "dark academia" so it sounds like you are in the film accuracy/authenticy camp(thats dope). For that you will need to hit thrift/antique stores. And you can find alot of stuff on etsy/ebay/facebook marketplace. Start with finding a Bentwood Steamer/cabin Trunk that you can refinish. Also find a "Chesterfield" loveseat. Rising Tide Design Co. makes very nice inexpensive leather Chesterfields in Wisconsin USA, but you may be able to find one used somewhere.

And yes, keep researching dark academia for ideas. Also try "Cottagecore" or "Dark cottagecore". I consider 12 Grimmauld Place to be a good mixture of dark academia/cottagecore.

Good luck and post pics

Oh my gosh, how did you know that I have spent the last week straight eyeballing the PBTeen site?! 👀👀

Those magical velvet quilts fill me with longing, I tell you!

https://www.pbteen.com/m/products/wizarding-world-magical-velvet-quilt-sham-faint-gray/?pkey=charry-potter-enchanted-bedroom&isx=0.0.295

And seeing that they sell the bedding in king size... I asked me boyfriend if he would mind if I decorated our entire place in “Harry Potter style” (once we get a place), and he served me with a big “Woman, what the f?” look.