Instructions
Go to the hunting missions in 25, past the Kolaakan Desert, or 27, the Mystery of the Desert's Sandstorm. Activate the Rain Miracle. This has a chance of bringing out Gancheeks, which look like purple snails. These creatures drop Kasu seeds and magical items when defeated.
Enter Mission 30-1, Protector of Mechaku Jungle, and at the end of it is the first appearance of Shookle, the plant boss from the first Patapon. When you attack the plant enough and it staggers, it drops Kasu seeds.
Defeat Shookle and you unlock 30-2 (Patapon Eating Plant Shookle). Here, Shookle is more likely to drop seeds, as he has more health and thus the fight is extended. Like the hunting missions, these can be repeated as much as the player likes, although the desert sequences aren't as difficult.