Leaves can be gathered from bushes or tufts as the game calls them.  As with all the resources, they can be gathered from either your city or your neighbor's city.  The tufts are also available from the store.  You may note that the cactus cluster is included in this grouping as well.  They do, in fact, give leaves not water.  And finally, there is a decorative fruit tree found in the market in the Ornamental Decorations that gives leaves when refreshed.