How to Get a Thunderstone in "Pokemon Crystal"by Seth Amery ; Updated September 22, 2017
Some Electric-type Pokemon need the Thunder evolutionary stone to evolve, most notably Pikachu and Eevee. Obtaining the Thunderstone isn't the biggest dilemma you'll face in "Pokemon Crystal"; in fact, it's a bigger struggle knowing when to use it. As the 14th episode of the animated series showed, it's okay to hang on to your Pokemon for a few more levels; in fact, you may benefit from delaying your Pokemon's evolution.
Getting the Thunderstone, Option One: Bill's Grandfather
Once you arrive in Fuchsia City, you'll see a house just beyond the large gym. Inside awaits Bill's grandfather, ready to hand over a Thunderstone to a trainer who shows him a Pichu. The only way to get a Pichu in "Pokemon Crystal" is to breed Pikachu or Raichu. You can find the daycare south of Goldenrod City after finding a wild Pikachu on Kanto Route 2.
Getting the Thunderstone, Option Two: Lass Dana
Heading to Johto Route 38 gives you the chance to battle Lass Dana, who will give you her phone number after defeating her Electric-type Flaafy and Water-type Psyduck. Occasionally, she'll call you and offer you a Thunderstone as a gift. This is random and unfortunately, there is no way to manually trigger the gift other than to keep playing.
Using the Thunderstone: Eevee or Pikachu
In "Pokemon Crystal," the only two Pokemon you'll get to use the Thunderstone on are Pikachu and Eevee. If you use one on Pikachu, it evolves into Raichu. Giving it to Eevee, on the other hand, evolves it into a Jolteon.
While you can give your Pokemon the Thunderstone at any time, evolving your Pokemon too early with a stone will cause him to miss out on essential moves, such as Pikachu's Thunder at level 41, which Raichu won't ever learn. Make sure your Pokemon has learned his entire moveset before evolving him with the Thunderstone.