Castle Treasury

At the beginning of each player's upkeep, that player puts a colorless artifact token named
Gold onto the battlefield. It has "Sacrifice this artifact: Add one mana of any color to your
mana pool."
{t}, Sacrifice X Gold tokens: Search target
player's library for a card with converted mana cost X and exile it. You may cast that card for {x} for as long as it remains exiled.
(X cannot be zero.)