Yes at last Avatar comics (and some books) are available to buy digitally.
This news comes from Dark Horse's own blog, http://www.darkhorse.com/Blog/1284/new-releases-digital-store-2-6-13 .
So if you want Avatar comics on your Ipod, Ipad or other like devices that is now possible because they are all up on Dark Horse's Digital Store .
Here is a list of the releases:
-Avatar Free Comic Book Day 2011 comic
-The Lost Adventures
-The Promise Part 1
-The Promise Part 2
-The Promise Part 3
-Avatar The Last Airbender Art Of The Animated Series
What this means going forward is that new Avatar releases (Like The Search Part 1 in March) will go up quicker on the store possibly same day as physical copies which should save some people finding a comic store. Same day release is NOT yes confirmed.
It also means if you by any chance miss out on the Free Comic Book Day 2013 Avatar comic on May 4th it will be available digitally also.