Whir of Invention is a rare Instant card from Aether Revolt (AER), collector number 49. Current market price: $5.49 (foil $12.49), TCGplayer market prices via Scryfall.
Improvise (Your artifacts can help cast this spell. Each artifact you tap after you're done activating mana abilities pays for {1}.) Search your library for an artifact card with mana value X or less, put it onto the battlefield, then shuffle.