Just went to Target's website, and they do have Avatar DVDs at their website.  In fact, I saw that they had a listing for Volume 3 of Book 2 (with a release date of May 22)!

http://www.target.com/gp/detail.html/sr=8-10/qid=1171242036/ref=sr_8_10/601-2402232-9902532?ie=UTF8&asin=B000NA21WW

Just popped by Amazon, and they corroborate this:

http://www.amazon.com/Vol-3-Book-Avatar-Last-Airbender/dp/B000NA21WW/sr=1-13/qid=1171242134/ref=sr_1_13/102-0398720-3372128?ie=UTF8&s=dvd

So, we can look forward to Book 2 Volume 2 on April 10, and Book 2 Volume 3 on May 22!  :)