Mark is looking for a good Bluetooth keyboard for his iPad Air and his smaller Android tablet. Leo says it would work with both, but he'll want to be sure he gets one that has a slot, and not one that doubles as a case. Microsoft makes a folding one that gets really tiny and expands. It's called the Universal Foldable Keyboard, and it's $100 list.
Leo recommends checking out The Wirecutter for tips and reviews.
Logitech's Easy Switch is a good one with great keys and good battery life. Another good choice is the Amigo, which he can learn more about at http://twit.tv/content/accessory-pick-lapworks-amigo-folding-keyboard.