It's available in the following areas: Route 4, 17, 18, Driftveil City,and P2 Laboratory
All of which you have to surf too. And you would have actually unlocked all of these areas by the time you obtain surf.
Route 4 - Lv. 5-15
Route 17 - Lv. 5-15
Route 18 - Lv. 5-15
Driftveil City - Lv. 10-25
P2 Lab. - Lv. 5-15
In short, your best bet would be Driftveil City maxing at level 25.
