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You can buy each season or individual episodes of ATLA Extras off of iTunes and amazon.
I'm not sure if you can find the episodes online, but the Avatar wiki has a list of all the extras from all episodes:
So you may not be able to watch them pop up on the screen, but you could read as you watch. If I find another way, I'll let you know!
EDIT: Yeah, you can buy and stream all of them on Amazon
Also available for purchase on YouTube's streaming service.
No, Last Airbender Plus is not available on DVD. It is however available on Amazon Video.
It's called Avatar Extras and you can find it on Amazon, Google Play and iTunes.
Looks like Amazon has it, but it's pretty pricey per season.
Avatar Extras. You may be able to find a torrent somewhere online, but officially you can buy them on Amazon.