Requirements:

1.must have a pokemnon that knows fly
2.must have pokemon that knows surf
3.must have visited both Cinnabar Island and Viridian city

Steps:
1.put the item you want more of third in the row of items. you do this by hitting the select button on the item and then scrolling to the third and switching
2.fly to Viridian city
3.talk to the old man who "taught" you how to catch pokemon.answer "no" to his question.
4.after he shows you, fly to Cinnabar Island immediatly
5.surf on the right side of Cinnabar (this means you should be barely touching land)
6.eventually you will run into a pokemon that either has a messed-up-name or is called missingno.
7.run from battle. you will know have an almost unlimited supply of the item.
IMPORTANT!-do not catch any of the pokemon listed above. they will mess up your game.