pretty much every where.
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You can't get nidoran in emerald only in firered and leafgreen.
Only On FireRed LeafGreen.
At The Pokemon Safari In Fuschia City
You can find Nidoran in route 3 (sometimes even Nidorino\Nidorina) and the Safari Zone in Fuchia city.
You can catch her on route 103, really rare though so look hard! :)