I did a search on iTunes also, and "Roar!" isn't showing up either in results or on Michael Giacchino's page. However, if you go to the original URL that links directly to "Roar!"'s page on iTunes, as of this writing it's still showing up as being available. And for $0.99, the same as most other tracks on iTunes (it was originally selling for $1.99).
It's probably just a glitch in the iTunes database. I can't think of any other reason why a track that was in such wild demand would be yanked so quickly, or even at all. Anyhoo, if ya go to this link you should still be able to get yer "Roar!" :-)