I CAN'T FIND A DARK CRYSTAL!!!!!!!!

I NEED HELP! I CAN'T FIND A DARK CRYSTAL IN KINGDOM HEARTS 2. IT'S SO STUPID. I NEED ONE TO COMPLETE THE ITEM LIST SO I CAN GET THE SEVENTH ORICHALCUM+ TO MAKE ULTIMA WEAPON SO I CAN BEAT SEPHIROTH AND GET FENRIR. FENRIR IS ACTUALLY AN ESPER FROM FF6, BUT THEY MADE IT INTO A KEYBLADE. I AM TRYING TO CONVINCE MY BROTHER TO LET ME USE ACTION REPLAY MAX TO GET IT, BUT HE WON'T LET ME. IF YOU NO HOW TO GET A DARK CRYSTAL, TELL ME!!!!!!!!!!!!!!!!!!