Each Pokemon has a preferred hiding spot. HEre's where to find many of them.

RED FIELD

Pallet Town: Charmander, Rattata, Nidoran (M), & Poliwag.
Viridan Forest: Weedle, Pidgey, & Pikachu.
Pewter City: Spearow, Jigglypuff, Ekans, & Magikarp.
Cerulean City: Oddish, Mankey, Jynx, & Abra.
Vermilion City Seaside: Shellder, Krabby, Ekans, & Farfetch'D.
Rock Mountain: Voltorbe, Diglett, & Mr.Mime.
Lavender Town: Gastly, Magnemite, Cubone, & Electabuzz.
Cycling Road: Spearow, Doduo, Lickitung, & Snorlax.
Safari Zone: Paras, Rhyhorn, & Chansey.
Seafoam Islands: Horsea, Staryu, Seel, & Articuno.
Cinnabar Island: Ponyta, Growlithe, Omanyte, & Kabuto.
Indigo Plateau: Machop, Onix, & Ditto.

BLUE FIELD

Viridan City: Squirtle, Nidoran (M), & Bulbasaur.
Viridian Forest: Caterpie, Rattata, & Pikachu.
Mt.Moon: Zubat, Paras, & Clefairy.
Cerulean City: Bellsprout, Meowth, Jynx, & Abra.
Vermilion City Streets: Shellder, Krabby, Farfetch'D, & Sandshrew.
Rock Mountain: Diglet, Voltorbe, & Mr.Mime.
Celadon City: Mankey, Meowth, Eevee, & Porygon.
Fuchsia City: Magikarp, Goldeen, Kangashan, & Exeggcute.
Safari Zone: Doduo, Nidoran (F), & Chansey.
Saffron City: Ekans, Sandshrew, Hitmonchan, & Hitmonlee.
Cinnabar Island: Ponyta, Koffing, Aerodactyl, & Magmar.
Indingo Plateau: Geodude, Ditto, Moltres, & Mewtwo
Note: There are more Pokémon in each area but those are the ones you will find most often.