Where did the 9 tails of Sora go?
Short version: He was just a filler character...

Long version:

Sora appears only in the Twelve Guardian Ninja arc of the anime. In the other arcs of the anime and in the manga he is not mentioned anywhere.

In other words, as readers and viewers of Naruto, we don't know if Nine-Tail's chakra, that left Sora's body, found a new host or did come back to Naruto, the real host of the Nine-Tails.

Links to external resources: Twelve Guardian Ninja (Arc), Sora