Ebay, me 10 years ago for £50, another game store...
What do you mean "ruined"? Because you could find one of those rarer disc-repairing shops.
You shouldn't check standard game stores, either, because FFVII is considered a classic today, harder to find in normal stores and at a higher price than many PS2 games. So you might be able to find it in one of those electronic/novelty collectors shops (like the ones which sell special edition consoles, posters, figures and the like).
Or you could go to a second-hand store.