Check Amazon reviews for the style you want. I wanted one that is NOT a back cover or folio. I favor the Logitech Ultrathin which is akin to a Microsoft Surface keyboard but with REAL keys (and in fact came out before Microsoft's).
IMPORTANT: There are TWO Logitech Ultrathin models for ipad Air (plus the various models for other iPads) and Amazon seems to sell the flawed OLD model. I had two of that original model and had to return both because the iPad Air would pry up the plastic in front of it, and looked to be heading towards a breakage. Plus, one was warped and wouldn't sit level.
The NEW model sounds better, though I have not tried it. You can fine YouTube reviews of it, and it's on Logitech's site, but I've never seen it for sale anywhere except directly from Apple. (Maybe that will change when it's been out longer?)
Here's how to spot the NEW model:
1. It has an adjustable-angle hinge (but this may make portrait use difficult--I wonder).
2. It has an extra row of key: real full-size number keys plus separate function keys.
3. It has big rubber bumpers (the size of keys) at the front corners to keep the iPad away from the keys. Therefore, the arrow keys "T" is NOT the last thing in the lower-right. The arrow keys are moved inward and that big bumper is what's at the very corner.
There are other keyboards I like the design of, but ONLY this one seems to lack obvious problems and have ALL the keys in the correct places, so your typing habits aren't screwed up. Some other great-looking designs sadly have relocated the quote keys or question mark or some other key you'll use many times a day. Deal-breaker for me.
And some of the Zagg ones look great (backlight!) except they grip the iPad tightly--which can leave marks, and require a lot of force to insert/remove. I want to attach/remove the keyboard easily, such as for portrait use, so that grip-hinge mechanism is unacceptable to me.