You can find the element of collections by harvesting the crops, feeding your animals, collecting the profit, chopping the trees and clearing the thickets etc. The elements drop randomly and it depends only on your luck. All the collections have "speaking" names, for example, elements of Barn collection you can get by collecting the profit from the Barn, Cherry collection - from your cherry trees, Forester`s collection - by chopping the trees etc. If you place mouse at any collection, you can see the prompt where you can find its elements. Don`t forget also that you can report your success in the newsfeed - just click "Share" button when you exchange the next collection. In this case your neighbors can use this report and get some bonus in the game.