According to this source players who purchase lets go pikachu or eevee from participating retailers in the month of may . can receive a serial code that can be used to redeem a shiny pikachu or shiny eevee depending on their games .
Those who already have the game , can still receive a code at the pokemon festa event in korea from May 3rd to May 12th . In order to get a code p;layers have to the show their switch with the game installed , the game cartridge or a pokeball plus to the event staff
Both pikachu and eevee will be at level 10
Pikachu knows thunderbolt , quick attack , iron tail and helping hand .
Eeevee knows double edge , dig , iron tail and helping hand .