The Logitech Ultrathin Keyboard Cover, the Belkin Qode Ultimate Pro Keyboard Case, and the ZAGG Rugged Book Detachable Keyboard and Case are all well-reviewed covers that come with keyboards for various iPad models. They all appear in the online Apple Store and many of the Apple Store retail locations.
As of 2015, prices range from $89.95 for the Logitech Ultrathin Keyboard Cover Mini for the iPad mini up to $149.95 for the Belkin Qode Ultimate Pro Keyboard Case for the iPad Air. The keyboards all feature a standard QWERTY layout and Bluetooth connectivity. Belkin also carries covers with wired keyboards.