Amazon gets their books from a wholesaler, who gets them from Bella Distribution. Generally, all books ship out on pallets to wholesalers and distributors 2-3 weeks prior to the release date, after they are released by the printer. The time it takes for those wholesalers to inventory, unpack, re-pick, pack and ship books out to their clients (booksellers and others) can take anywhere between 7-10 days (generally).
In the case of Amazon and other large retailers, the distributor shipments are then sent to their own internal warehouse hubs where the same process happens again: books are unpacked and repacked and shipped to their individual stores and distribution centers, where they are unpacked and logged in as available. So availability at these locations can vary greatly, depending on their own internal processes.
On the other hand, books are shipped to independent bookstores and online retailers about a week prior to release date. Because the shipment is direct, these sites receive the books quickly and usually have them in stock, ready to mail out, before Amazon or most of the other large retailers.
So if you can’t wait for a book, your local independent bookstore, Bella Books will get it for you the quickest.