To get 99 of any item, first put the item to be multiplied in the sixth slot. Then, fly 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, answer "No". After he explains how to catch a Weedle, fly to Fuchsia City. Make your way south, then when you reach the water's edge, use Surf and go to Seafoam Island. After arriving, swim up and down the beach until running into Pokemon "M". Then, defeat "M" or run away, but do not catch him. You should now have 99 of your sixth item.
You can do this as many times as you would like!

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