Where to find Poliwhirl in a hidden grotto?

Question

and how often do I encounter Pokemon

Answer 1

Hidden Grottos List

Pinwheel Forest, the second one on that list (there are two grottos in Pinwheel). The chances of finding a Pokemon in any grotto is 20%, and the chance of finding a Poliwhirl in that grotto is 1%.

Answer 2

Pinwheel Forest. In the outer area.

http://bulbapedia.bulbagarden.net/wiki/Pinwheel_Forest#Outer_area_3