Bulbasaur Forested areas, probably. Jungles, maybe. (Viridian Forest? Safari Zone?)
Charmander Probably... caves and warmer places. (Since there isn't a fire-oriented area in Kanto mainland, probably Rock Tunnel. I suppose Mt. Ember in the Sevii Islands works as well.)
Squirtle Probably anywhere watery. (Specifically in the Kanto region, I'd bet off the coasts of Viridian, Pallet, Cinnabar, and Fuchsia, injcluding the Seafoam Islands.)
Chikorita Like Bulbasaur, forested, plant-filled areas. (Ilex Forest, National Park, Safari Zone.)
Cyndaquil Probably darker areas (their eyesight probably weakened due to not needing to see) and warmer areas. (Mt. Mortar, Dark Cave.)
Totodile Given that it's a crocodile, would probably live in shallower waters. (Union Cave, off the coast of Olivine, Whirl Islands, etc.)
Treecko Tree-filled areas. (Petalburg Woods.)
Torchic Warm, dry places. Given that it's a bird, probably wooded areas, too. (Mt. Chimney, Petalburg Woods.)
Mudkip Wet areas, probably also rocky caves. Thankfully, there are tons of those. (Shoal Cave, Seafloor Cavern, perhaps Meteor Falls.)
Turtwig Wooded, grassier areas just like every other freakin' grass type out there. (Eterna Forest, Flower Paradise, Lake Verity/Valor, perhaps.)
Chimchar Warmer, dryer climates. Probably also wooded areas, given it's a monkey. (Eterna Forest, Mr. Coronet, Stark Mountain, perhaps Lake Valor/Verity.)
Piplup Wetter areas, and probably colder climates (since it's a penguin). (Lake Acuity, off the coast of Sandgem.)