Whir of Invention is a rare Instant card from Secret Lair Drop (SLD), collector number 311. Current market price: $9.30 (foil $11.06), 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.