I have a skeleton farm and I enchanted DOZENS of items already. I never saw "mending" though. The Wiki says, that you find this on the lower enchanting levels, but I never saw it.
Note: (Normally) Before I enchant an item, I look through all of them and check on each what I would get: Helmet, Chestplate, Leggings, Boots, Sword, Bow. I think chances that I really never could have seen it are pretty low.
Can you get this enchantment only from the end and from books? I saw some enchanted items in the end cities with mending.