The following links are to sites that you will be able to watch on:
Sidereel:
http://www.sidereel.com/Avatar%3A_The_Last_Airbender
Netflix: Instant
Book 1:
http://www.netflix.com/WiMovie/Avatar-The-Last-Airbender-Book-1/70043989?strackid=19af8f4692d15590_0_srl&strkid=613622931_0_0&trkid=438381
Book 2:
http://www.netflix.com/WiMovie/Avatar-The-Last-Airbender-Book-2/70059201?strackid=19af8f4692d15590_2_srl&strkid=613622931_2_0&trkid=438381
http://www.netflix.com/WiMovie/Avatar-The-Last-Airbender-Book-3/70077825?strackid=19af8f4692d15590_1_srl&strkid=613622931_1_0&trkid=438381
if anyone else knows any other links, post them here.