From Bulbapedia, the community-driven Pokémon encyclopedia.
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Weezing cannot be found in the wild in this game, but its pre-evolution, Koffing, can, and is available in the Burned Tower.
Koffing is also available in limited quantity in the Team Rocket HQ in Mahogany Town.
a shiny hunting tip
I believe it is a good idea to list the poke balls you should use and places to go without moves like teleport, explosion, ect.