I had the same problem, looked everywhere on my iBooks app on both laptop and iPad and eventually found the order number in my iTunes Store.
1) Sign up into your iTunes account (iTunes Store) on your MacBook.
2) Go to the tab "Store" on top of your screen.
3) Click "View Account".
4) The iTunes Store Tab will open.
5) Scroll down to "Purchase History".
6) It should view "Most Recent Purchase" and "View All".
7) Click "View All" and you'll see order numbers of the items you purchased.
Your Purchase History may be hidden in your iBooks tab so this is probably your best bet. Also "See your purchase history in the iTunes Store - Apple Support" or "Hide and unhide purchases in the Mac App Store - Apple Support " for detailed steps with pictures.
*Not sure when they give the actual order number for pre-orders. For me it showed up 4 days in my Purchase History before the actual release day. It may vary for each order.