There are several areas that Pikachu is now spawning. These are at
East Coast Park Area H
Kent Ridge Park
Yishun Reservoir Park
Yishun East Park
Sembawang Golf Course
Head to the Mount Faber hills to farm Charmander. This is great slope training at the same time!
For Charmander too there also appears to be a mini nest at the Nicoll Highway Open Field, Beach Road and Republic Avenue areas. So perhaps you can try out these places as well.
To farm Drowzee Pokemons, head to East Coast Park Zone B.
To farm Meowth Pokemon you can head to the NSRCC (National Service Resort and Country Club) located at the East Coast Park Area H.