These Evolution Stones need to be purchased in the game at specific vendors. One vendor that sells Evolution Stones is the one in Celadon City. You will not be able to go there straight away. You'll need to progress through the story of the game a bit before you get there. The Department Store in the city will sell you the stones that you need to evolve. They will cost you 5000 Poké Dollars for each stone that you want to purchase. All of the evolution stones can be purchased here, except for the Moon Stone. That will need to be found on Mt. Moon.
How to Mega Evolve
Almost all Pokémon can evolve in some form or another, but a smaller selection of them can evolve even further with a Mega Evolution. In order to do a mega evolution you must get the Key Stone and the Mega Stone for that specific Pokémon. These stones can be found while playing the story of Pokémon Let's Go before you challenge the Viridian Gym.
How to Evolve Pikachu and Eevee
Your Pokemon partner (either Pikachu or Eevee) can not be evolved, but other Eevee or Pikachu Pokémon that you catch in the world can be.