How do I choose Eevee's evolution?

Your starter in the Let's Go game can't evolve.
All the stones are sold in the department store in Celadon city for 5000.

Free stones:

• Fire stone- Pokemon Mansion on Cinnabar Island
• Water stone- Route 12
• Thunder stone- Power Plant

If you mean the Eevee you get as a starter, it can't evolve at all.

Any other Eevee evolves by its usual method: Fire Stone for Flareon, Water Stone for Vaporeon, and Thunder Stone for Jolteon. All of the elemental evolution stones can be bought at the Celadon Department Store.