I don't really know about Pokemon leafgreen, but in Pokemon red you can get it in seafoam islands. Hope this helps!
You can't. Try FireRed and LeafGreen.
migrate from firered/leafgreen
Sorry, but you have to trade it from fire red.
I know firered can, if you seen a shellder look at it on your pokedex and check its area if it says unkown you will HAVE to trade a shellder to leafgreen from firered.
skyther, chanze, shelder, staru, nidogween
Piplup does not exist in Pokemon LeafGreen or FireRed.
No, you cannot find cyndaquil in LeafGreen.
you should go to gamestop
LeafGreen version does not have Growlithe.
In the grass...
You cannot catch houndour in Pokemon leafgreen either get one from Pokemon emerald or Pokemon colosseum.
Do you mean shelder? To find shelder, surf around Cinnabar Island. You can also find Dewgongs, but they are rare.