You'll have to be almost completely done with the game to do this cheat, and make sure you hve a flying Pokemon with the technique FLY. Go to Viridian City and talk to the man that had his coffee and select the option [No]. Then he'll show you how to catch a Pokemon. After the review, FLY to Fushia City. Put the item you want to duplicate 6th on the ITEM menu (use the SELECT button). Use a water Pokemon with the SURF ability and swim on the right side shore of the island. Swim up and down the coast until you encounter a Missingno. or a Missing M (you will also encounter Pokemon levels 100-186 but will go back to level 100 after "gaining" a level). Defeat one and the sixth item in your inventory will have an image next to it, which means you have over 200 copies of that item. I prefer to copy Rare candies so that I can make my Pokemon to Level 100 without training but this ruins the fun of the game.