Up to now, all my Avatar-related minis have been from The Legend of Korra, but I haven't forgotten the original series.
In the original series, the gang was mostly kids and early teens, so finding minis that aren't too big or too small is surprisingly hard.
For Katara, I found the Dungeons and Dragons Earth Shugenja, which has a good pose for a waterbender. It doesn't quite have the hair loopies, but I doubt any miniature does. I tried to make some but they looked pretty bad, so I decided to skip it.
As a side note, this mini makes the 3rd miniature I've done for a character voiced by Mae Whitman.