Cause I know you don't mean to ask where you can get Graviboard for free without paying for it, right? Cause that would be piracy, which any discussion of is against this forum's rules ... and illegal!
Given that, I'm sure you meant to ask where you can find and purchase a legitimate version. So, it's as The Archangel said - you can buy it from the jail breaker's "app store" Cydia for a mere $2.99 (USD).