If you haven't started your journey yet, you can actually skip the standard starters (Bulbasaur, Charmander, and Squirtle) to get Pikachu :
Check if your costumed Pikachu can evolve. Some event versions are locked, while others allow you to evolve them into a costumed Raichu. Pro-Tips for Pikachu Fans PokГ©mon GO Pikachu
Most costumed Pikachu can be Shiny. These are among the rarest items for trading in the game. If you haven't started your journey yet, you
Look out for "Hat" Pikachus during Halloween, the Holiday season, and Pokémon Day. These are among the rarest items for trading in the game
On the fourth or fifth reappearance, alongside the original trio, ready for you to catch! Hunting for Costumed Pikachu
During specific events, taking snapshots of your Pokémon might trigger a "photobomb" where a wild Pikachu appears on the map for you to catch.