#1 Get all 150 Pokemon without a game shark/genie #2 Get every single item in Seafom Island. There's even a hidden excape rope neer Articuno that you need to get. #3 Than use the excape rop you just found. The last place you had to be was in Cinnabar. #4 Than you have to realease all the stones that you have on you. You have to had a Thunder stone. #5 Than go back to Seafom Island caves. Use your item finder and you should find it.
It might take you a few trys to locate the stone. When you find it I would duplicate it because you can evolve ALOT of Pokemon into new forms!