You can easily purchase a Habitat for your Islands by following these steps:
1) Tap on Shop Icon in the bottom right hand corner
2) Tap on Habitats
3) Browse through and choose the one you need
4) Select where you wish to place it

NOTE -> Each time you purchase another of the same Habitat type, the price to purchase this will increase.
Some Habitats may need to be unlocked with help from your friends.