There are two ways -
- In the Dreamyard, not Striaton City, there is a girl standing near the lower right of the Cut tree. Talk to her, and she will ask you if you want one of the Pans, depending on your starter.
Snivy - Panpour
Tepig - Pansage
Ohsawott - Pansear
And the other ways is to catch them, as mentined before, in rustling grass in Lostlorn Forest and Pinwheel Forest (Inside only)