Ok, I'm using a Game Guide and it says that after the "event" at Memory's Skyscraper I acquire the Final Form.
I have yet to see it? It did say it happens randomly like Anti-Form. So I guess I just have to keep using Forms until it comes up? I didn't get a little message saying "you have acquired Final Form" like with the other ones.
Edit: Nevermind, it's random like Anti-Form.