When do you get a better keyblade in Kingdom Hearts?

Answer 1

As you progress through the story, characters will give you keychain that give your keyblades different forms.

you should get a keychain from every world. Some keychains are achieved in different ways than just being given to you. After the Monstro world is complete, go to traverse two and you should be given a keyblade when you meet Gepetto and pinnochio. The Oblivion keyblade is in a chest just above where you verse Riku the second time in hollow bastion.

And the ultimate blade is forged through synthesis by gathering rare materials.

I think you might get one from defeating Sephiroth too.