There are a total of 4 critter plants, and they are traded from the professor in 4 different spots. Northwest Tradepost, Desert Oasis, Forest Glade, and the Tremeltree. You need to have beaten the boss at Tremel for the professor to show, then just trade the strange seed pods you get from gnarts overworld ability and you'll get the four seeds. Check the video to show where the locations are, hope it helps!