How to get avatar ? And who sell some avatar ?

You can get avi item by fighting the bosses in the festival or buy some from the festival shop (you nead the specific curencie from the fest), the avatar shop (diamond only), the trade broaker, the item auction section and some Npc in the towns/ astryl's vale exploration zone.

PS: the item are not the same depending on where you got it, those who came from past festivals are rarer but can be find in retired avatar item in the festival shop of the right festival