No not randomly, it appeaers when you next go into anti form. (after the Sora vs Roxas scene)
If you really want final form do this.
-Enter World
-Drive (without enemies, this way the 'anticount' goes up without having to do anything)
-Leave World whilst still in drive (you get back the lost drive bars)
-Re-enter World
-Repeat 10 times or so
-Go into battle and drive, your anticount should be high enough to ensure antiform, however you will instead gain final form)
if you are unaware of what the anticount is, it is basically an invisible meter which goes up by 1 each time you drive. When there are enemies about when you drive the 'anticount' will make you go anti if it is high enough (although there is SOME chance involved)