How to get the mending enchantment?

Mending is a treasure enchantment, which you can only find in the world, you can not create it yourself on an enchanting table.

Treasure enchantments can be obtained by

• Fishing
• Chests in dungeons, temples, mineshafts, End cities etc.
• Trading (villagers charge double price for treasure enchantments)

Mending can be found on many items, including, for example, Fishing Rods.

Sources: 1.9 changelog, enchantment mechanics, trading pages.