The Legend of Korra Air Book has finally been released on DVD. There has been a lot of speculation on its release date but it is available to US citizens via Amazon.
Unfortunately, it is not available to fans outside the US yet, but for those of you who live in the US it'll make a great last-minute Christmas present for family, friends or yourself :)
Also, there is a trailer promoting the DVD, though it is the same as the one promoting the series.
Individual episodes and the entire series can also be downloaded from Amazon, while individual episodes are also available on DVD.