LEGO and Barnes & Noble have been pretty quiet about the upcoming Bricktober 2018 promotion for the Harry Potter (5005254) set but I have managed to get bits of details about how to get it. Some people have already managed to find them in stores and purchase them by themselves. They were found at or around the small LEGO section. There’s supposedly a promotion that is attached to it but there are no details for that.
I also heard that you could also ask an employee to see if you can put in a order them. The UPC is 673419294300 and it’ll only work if the warehouse for your store already has them allocated. I could not order it yet but I was told to check back regularly. The price on it was $7.99 each.