Question

Where do you get the Oval Stone in Pokemon Black and White?

Answer 1

Non-repeatable
- Challenger's Cave (B2F, requires Surf)

Repeatable
- Black City or White Forest from Carlos
- Dust clouds in caves
- Held by wild Happiny (50% chance)

Oval Stone (Bulbapedia)

Answer 2

Here:
Challenger's Cave (B2F)
Shop: Black City

Source