Maples leaves are items which are harvested from Maple Trees, which are dotted across the wilderness.
To harvest, you simply right-click a maple tree and from the context menu choose 'Pick Leaves'. If you do not see such an option, you either clicked the wrong type of tree, or someone has already harvested all the leaves before you.
Each tree can only yield 2 Maple Leaves before being depleted and needs to wait to respawn. As of the recent world wipe, maple leaves respawn on trees, based on decay ticks. This means that respawn is very much random.