In Mt. Moon, there is a Super Nerd that is protecting his newly found fossils from Team Rocket. He fights you. When you are victorious, he asks you which fossil you want. Select the Helix Fossil. Now, once you get to Cinnabar Island (Which is a LONG ways from now), head over to the lab and give the Helix Fossil to one of the scientists. Exit the building and re-enter to obtain Omanyte.
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Find a fossil underground. I did.
Piplup does not exist in Pokemon LeafGreen or FireRed.
You cannot catch houndour in Pokemon leafgreen either get one from Pokemon emerald or Pokemon colosseum.
That item doesn't exist in leafgreen.
Nowhere.