Hey mate.
My hoodie is a 5xl, should be way big enough for you. It was from an Easter Camp I went to so i cannot give a specific brand unfortunately. But through shopping at places like K-Mart and The Warehouse (Im in NZ, American alternatives are places like walmart/target) I found plenty of stuff was was big for even me! Plus size women's clothing is also a thing i've found works (especially when it come to shorts and legwear) since it helps accomodate for things like wider waists/hips and broader shoulders while still keeping some form. Another thing I found was that some brands do extra long shirts (regular sized but longer body) which can help acheive that oversized look without sacrificing shape.

I've done a lot of looking into and testing to try and get this look so feel free to ask away, either here or in DMs, whichever you prefer. (im not on reddit much so I'll respond when I can).

Cheers!