Loki is the God of Mischief. It's his job to fuck with good people's lives in unpredictable ways. I don't see him having a lot of trouble finding kryptonite if he needs it. The history of Krypton means there's a debris field full of it floating around in space somewhere.
You say that like it's a natural conclusion that rocks from your homeworld are somehow super poisonous to you and will strip you of your powers while you're near it.