If you frequently buy brand new Apple products, you know the cycle: initial pre-order stock sells out quickly, launch day stock goes fast, and then Apple starts offering next-day reservations for in-store pickup. The iPad mini has reached the last stage in the cycle.
Apple retail stores get a limited supply of new products (in this case, the iPad mini) each night. If you’re having trouble getting your hands on an iPad mini, your best bet is to order one through Apple’s online store and see if you can walk in and pick it up in person the next day at your local Apple Store.