The bad news is in. A few months ago, Tamat posted that Aboriginal Sivir (in game: Huntress Sivir), an unreleased skin at the time, would soon be added to the in game store and was erroneously held up due to issues with the store. Two months later, he offers us an apology for that miscommunication and confirms the only way to obtain this skin is by purchasing the Gamer's Choice Pack. This pack unlocks Huntress Sivir, a 10 win XP boost, and 20 champions for 3900 Riot Points. You can find it by navigating to the "Bundles" tab in the store and purchasing it as you would any other item.
"... It appears as if there was some miscommunication about how the Aboriginal Sivir skin can be acquired. Currently you can only purchase this skin by purchasing the Gamer's Choice Pack. Sorry about that!"Tamat, Lead Community Manager, via the official forums