There are four moonstones to be had in the game. One of them is obtained by buying stuff from Stiltzkin. If you've bought all the stuff from him in discs 1-3, in disc 4 he will have a Moonstone, a Ruby, and an Elixir for 5,555 gil. If you did not buy his stuff in Bran Bal, then I don't believe you will be able to buy this Moonstone in disc 4. That's the only thing you screwed up.