Beseech the Mirror

Bargain (You may sacrifice an artifact,
enchantment, or token as you cast this spell.)
Search your library for a card, exile it
face down, then shuffle. If this spell was
bargained, you may cast the exiled card
without paying its mana cost if that spell's
mana value is 4 or less. Put the exiled card
into your hand if it wasn’t cast this way.