99 of Any Item
To get 99 of any item, first put the item you want to multiply in the sixth item slot. Now use FLY and go to Viridian City. Talk to the old man almost all the way north of the town. When he asks if you are in a hurry, say no and watch him catch a weedle.
Right after that, fly to Fuchsia city. Make your way south and, when you you reach the water's edge, use SURF and make your way to Seafoam island.
When you get to seafoam island, swim up and down the beach until you run into a Pokemon called "M" (a glitch like Missingno.) Once you see him, kill him or run away. DO NOT CATCH HIM! After that, you should have 99 of your sixth item!