Just to add to the solution above.
You will first be able to catch Butterflies during Sallys mission Ex Cathedra.
Although you have to catch 40 for the achievement, if you have the skill Picky Picky which gives one extra resource then you only have to collect twenty as catching 1 will actually give you 2
You have to collect ten for the mission above so by the time you leave the train station you will in fact have twenty. For the ten just run around Luds Holm fields you find them around a plant its a non collectible plant.