Tap the icon with three dots in the bottom-left corner and select the icon with a chest in the menu that will appear on the bottom of the screen. This is the list of all the Collections in the game. Here you can exchange the Collections and see the hints about place where to find a Collection and a reward for exchanging it. The rewards are listed in the top left corner of a Collection, the hints where to find a Collection are written under the list of Collection’s items.
You can look for collection items yourself or send your zombie workers (Treasure Hunters and Fishermen). You can find collections items on the Big Land and on the Archipelago. 
Buy a Moneybox (Market >> tab with arrows), tap on it or on the icon with blue arrows in the bottom right corner, select the golden shovel icon and tap the objects or swipe through the field where you want to search.
You can buy Treasure Hunters and Fishermen in the Zombies tab of the Market. The zombie workers will bring the items from different sets every half an hour (Treasure Hunters) or hour (Fishermen). Make sure you collect the loot frequently!