Keep getting in to random battles around Winhill until PuPu appears. Then, do the same on Mandy Beach (South East of Timber), then again in the Kashkabald Desert (North of the Cactuar Island)
there's a fourth spot where you have to encounter the UFO before going to the forest, its a small island right around the northern peninsula on the right side of trabia, past the garden and chocobo forest (i believe). you can only get to it using the ragnarok.

once you go to all four spots, then you can go to grandidi forest, the area around the chocobo forest there, and well, everyone knows the rest