99 Of Any Item
Fly to Viridean city and talk to the man who teaches you how to catch pokemon. Respond "no" to his question. When he is done talking switch which ever item you want 99 of to the sixth position from the top. Fly to Cinnabar Island. Walk to your right. When you reach the coast surf up and down it until you find a pokemon named either "M" or "Missingno". Defeat them or run. Now your sixth item should have a symbol and a number next to it. Now you should have more then 99 of this item.
Go to the hotel for people in Celedon City. Walk up to the counter and turn right and walk to the wall. Go down two spaces and then up one. Press the "A" botton and the PC menu will apear. Try it.